Treat it exactly like Linux since they both use GNU libc.

rtree_node_init spinlocks the node, allocates, and then sets the node.
This is under heavy contention at the top of the tree if many threads
start to allocate at the same time.

Instead, take a per-rtree sleeping mutex to reduce spinning.  Tested
both pthreads and osx OSSpinLock, and both reduce spinning adequately

Previous benchmark time:
./ttest1 500 100
~15s

New benchmark time:
./ttest1 500 100
.57s

This resolves #485.

OS X 10.12 deprecated OSSpinLock; os_unfair_lock is the recommended
replacement.

Fix zone_force_unlock() to reinitialize, rather than unlocking mutexes,
since OS X 10.12 cannot tolerate a child unlocking mutexes that were
locked by its parent.

Refactor; this was a side effect of experimenting with zone
{de,re}registration during fork(2).

_exit(2) is async-signal-safe, whereas exit(3) is not.

Monitoring thread creation is unimplemented for Windows, which means
lazy-lock cannot function correctly.

This resolves #310.

Fix and clean up various malloc_stats_print() issues caused by
0ba5b9b6 (Add "J" (JSON) support to
malloc_stats_print().).

This resolves #474.

This resolves #480.

This resolves #396.

The raw clock variant is slow (even relative to plain CLOCK_MONOTONIC),
whereas the coarse clock variant is faster than CLOCK_MONOTONIC, but
still has resolution (~1ms) that is adequate for our purposes.

This resolves #479.

Some applications wrap various system calls, and if they call the
allocator in their wrappers, unexpected reentry can result.  This is not
a general solution (many other syscalls are spread throughout the code),
but this resolves a bootstrapping issue that is apparently common.

This resolves #443.

This works around malloc_conf not being properly initialized by at least
the cygwin toolchain.  Prior build system changes to use
-Wl,--[no-]whole-archive may be necessary for malloc_conf resolution to
work properly as a non-weak symbol (not tested).

This is generally correct (no need for weak symbols since no jemalloc
library is involved in the link phase), and avoids linking problems
(apparently unininitialized non-NULL malloc_conf) when using cygwin with
gcc.

glibc defines its malloc implementation with several weak and strong
symbols:

strong_alias (__libc_calloc, __calloc) weak_alias (__libc_calloc, calloc)
strong_alias (__libc_free, __cfree) weak_alias (__libc_free, cfree)
strong_alias (__libc_free, __free) strong_alias (__libc_free, free)
strong_alias (__libc_malloc, __malloc) strong_alias (__libc_malloc, malloc)

The issue is not with the weak symbols, but that other parts of glibc
depend on __libc_malloc explicitly.  Defining them in terms of jemalloc
API's allows the linker to drop glibc's malloc.o completely from the link,
and static linking no longer results in symbol collisions.

Another wrinkle: jemalloc during initialization calls sysconf to
get the number of CPU's.  GLIBC allocates for the first time before
setting up isspace (and other related) tables, which are used by
sysconf.  Instead, use the pthread API to get the number of
CPUs with GLIBC, which seems to work.

This resolves #442.

This is intended to drop memory usage to a level that AppVeyor test
instances can handle.

This resolves #393.

This resolves #393.

This resolves #393.

Use the correct level metadata when allocating child nodes so that leaf
nodes don't end up over-sized (2^16 elements vs 2^4 elements).

This avoids warnings in some cases, and is otherwise generally good
hygiene.

Rather than relying on two's complement negation for alignment mask
generation, use bitwise not and addition.  This dodges warnings from
MSVC, and should be strength-reduced by compiler optimization anyway.

This fixes warnings when building with MSVC.

Conditionalize use of --whole-archive on the platform plus compiler,
rather than on the ABI.  This fixes a regression caused by
7b24c6e5 (Use --whole-archive when
linking integration tests on MinGW.).

This reverts 13473c7c, which was
intended to work around bootstrapping issues when linking statically.
However, this actually causes problems in various other configurations,
so this reversion may force a future fix for the underlying problem, if
it still exists.

Prior to this change, the malloc_conf weak symbol provided by the
jemalloc dynamic library is always used, even if the application
provides a malloc_conf symbol.  Use the --whole-archive linker option
to allow the weak symbol to be overridden.

Refactor tsd so that tsdn_fetch() does not trigger allocation, since
allocation could cause infinite recursion.

This resolves #458.

Rather than protecting dss operations with a mutex, use atomic
operations.  This has negligible impact on synchronization overhead
during typical dss allocation, but is a substantial improvement for
extent_in_dss() and the newly added extent_dss_mergeable(), which can be
called multiple times during extent deallocations.

This change also has the advantage of avoiding tsd in deallocation paths
associated with purging, which resolves potential deadlocks during
thread exit due to attempted tsd resurrection.

This resolves #425.

Add spin_t and spin_{init,adaptive}(), which provide a simple
abstraction for adaptive spinning.

Adaptively spin during busy waits in bootstrapping and rtree node
initialization.

Remove mallctls:
- opt.lg_chunk
- stats.cactive

This resolves #464.

Make decay-based purging the default (and only) mode.

Remove associated mallctls:
- opt.purge
- opt.lg_dirty_mult
- arena.<i>.lg_dirty_mult
- arenas.lg_dirty_mult
- stats.arenas.<i>.lg_dirty_mult

This resolves #385.

Simplify decay-based purging attempts to only be triggered when the
epoch is advanced, rather than every time purgeable memory increases.
In a correctly functioning system (not previously the case; see below),
this only causes a behavior difference if during subsequent purge
attempts the least recently used (LRU) purgeable memory extent is
initially too large to be purged, but that memory is reused between
attempts and one or more of the next LRU purgeable memory extents are
small enough to be purged.  In practice this is an arbitrary behavior
change that is within the set of acceptable behaviors.

As for the purging fix, assure that arena->decay.ndirty is recorded
*after* the epoch advance and associated purging occurs.  Prior to this
fix, it was possible for purging during epoch advance to cause a
substantially underrepresentative (arena->ndirty - arena->decay.ndirty),
i.e. the number of dirty pages attributed to the current epoch was too
low, and a series of unintended purges could result.  This fix is also
relevant in the context of the simplification described above, but the
bug's impact would be limited to over-purging at epoch advances.