Hi guys,

gthread-posix.c contains the following check to determine whether or not native mutex support can be enabled : 

===================
/* clang defines __ATOMIC_SEQ_CST but doesn't support the GCC extension */
#if defined(HAVE_FUTEX) && defined(__ATOMIC_SEQ_CST) && !defined(__clang__)
#define USE_NATIVE_MUTEX
#endif
===================

However, some architectures - specifically sparc v8, but possibly others - define __ATOMIC_SEQ_CST even though they do not support the full set of atomic instructions. This leads to the following compilation error. 

===================
  CC       gspawn.lo
  CC       gtester.o
  GEN      glib-public-headers.txt
  CCLD     libglib-2.0.la
  CCLD     gtester
./.libs/libglib-2.0.so: undefined reference to `__atomic_compare_exchange_4'
collect2: error: ld returned 1 exit status
Makefile:1439: recipe for target 'gtester' failed
make[6]: *** [gtester] Error 1
Makefile:2455: recipe for target 'all-recursive' failed
make[5]: *** [all-recursive] Error 1
Makefile:1168: recipe for target 'all' failed
make[4]: *** [all] Error 2
Makefile:1207: recipe for target 'all-recursive' failed
make[3]: *** [all-recursive] Error 1
Makefile:849: recipe for target 'all' failed
make[2]: *** [all] Error 2

===================

The choices are to either allow the compiler to use libatomic to substitute the missing instructions; or to allow the application to determine what to do at compile time. 

My guess is the best fix is for glib to detect this and disable USE_NATIVE_MUTEX. I propose that an additional check be added for __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4, since we seem to need __atomic_compare_exchange_4(). With this patch applied, libglib2 compiles successfully.