build: Fix test for G_ATOMIC_LOCK_FREE in meson.build

Commit 3e96523e6b did not entirely fix the test, as the compiled test
code did not have a main() function, so failed to link with:

 /usr/lib/gcc/x86_64-redhat-linux/7/../../../../lib64/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

This caused an invalid mixtures of builtin and non-builtin atomics/locks
to be used, which caused deadlocks in a number of tests.

Fix the atomic ops test in meson.build, and the unit tests all start
working again.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=796164

meson.build

@@ -1460,9 +1460,10 @@ glibconfig_conf.set('G_ATOMIC_OP_MEMORY_BARRIER_NEEDED', glib_memory_barrier_nee
 # to abort and advise the user to fix their CFLAGS.  It's better to do
 # that then to silently fall back on emulated atomic ops just because
 # the user had the wrong build environment.
-atomictest = '''void func() {
+atomictest = '''int main() {
   volatile int atomic = 2;
   __sync_bool_compare_and_swap (&atomic, 2, 3);
+  return 0;
 }
 '''
 if cc.links(atomictest, name : 'atomic ops')