Use only one aligned alloc func, work around msys2

As recommended by @nirbheek, only search for a single aligned memory
allocation function: ebassi#86 (comment)

The cascade of tests means we can exclude the ones for which MSYS2's
native mingw-w64 builtins and headers conspire to provide meson with a
dud test. Although it is still not clear that meson is doing the right
thing in this case (mesonbuild/meson#1083), we can work around the
failed build in graphene by excluding certain POSIXy cases on Windows.

Closes ebassi#76, leaving ebassi#88 unsolved.

meson.build

@@ -129,10 +129,20 @@ conf.set('HAVE_STDBOOL_H', cc.has_header('stdbool.h'))
 conf.set('HAVE_MEMORY_H', cc.has_header('memory.h'))
 
 # Functions
-conf.set('HAVE_ALIGNED_ALLOC', cc.has_function('aligned_alloc', prefix: '#include <stdlib.h>'))
-conf.set('HAVE__ALIGNED_MALLOC', cc.has_function('_aligned_malloc', prefix: '#include <malloc.h>'))
-conf.set('HAVE_MEMALIGN', cc.has_function('memalign', prefix: '#include <stdlib.h>\n#include <malloc.h>'))
-conf.set('HAVE_POSIX_MEMALIGN', cc.has_function('posix_memalign', prefix: '#include <stdlib.h>'))
+if cc.has_function('memalign', prefix: '#include <stdlib.h>\n#include <malloc.h>')
+  conf.set('HAVE_MEMALIGN', 1)
+elif cc.has_function('_aligned_malloc', prefix: '#include <malloc.h>')
+  conf.set('HAVE__ALIGNED_MALLOC', 1)
+elif not (host_system == 'windows')
+  # These tests have proven unreliable on MSYS2 because meson falls
+  # back to testing compiler builtins instead of what we asked for.
+  if cc.has_function('aligned_alloc', prefix: '#include <stdlib.h>')
+    conf.set('HAVE_ALIGNED_ALLOC', 1)
+  elif cc.has_function('posix_memalign', prefix: '#include <stdlib.h>')
+    conf.set('HAVE_POSIX_MEMALIGN', 1)
+  endif
+endif
+
 conf.set('HAVE_SINCOSF', cc.has_function('sincosf', prefix: '#define _GNU_SOURCE\n#include <math.h>'))
 
 # Debugging