Fix librt detection when using a Cray compiler wrapper

The Cray compiler wrappers will often add `-lrt` to the base compiler with
`-static` linking (the default at most sites.) However, `-lrt` isn't
automatically added with `-dynamic`. This means that if jemalloc was built with
`-static`, but then used in a program with `-dynamic` jemalloc won't have
detected that librt is a dependency.

The integration and stress tests use -dynamic, which is causing undefined
references to clock_gettime().

This just adds an extra check for librt (ignoring the autoconf cache) with
`-dynamic` thrown. It also stops filtering librt from the integration tests.

With this `make check` passes for:
 - PrgEnv-gnu
 - PrgEnv-intel
 - PrgEnv-pgi

PrgEnv-cray still needs more work (will be in a separate patch.)