Restructure *CFLAGS/*CXXFLAGS configuration.

Convert CFLAGS/CXXFLAGS to be concatenations:

  CFLAGS := CONFIGURE_CFLAGS SPECIFIED_CFLAGS EXTRA_CFLAGS
  CXXFLAGS := CONFIGURE_CXXFLAGS SPECIFIED_CXXFLAGS EXTRA_CXXFLAGS

This ordering makes it possible to override the flags set by the
configure script both during and after configuration, with
CFLAGS/CXXFLAGS and EXTRA_CFLAGS/EXTRA_CXXFLAGS, respectively.

This resolves #504.