meson: Fix checks for posix_memalign and stpcpy

The 'no-builtin' checks were just plain wrong. For accurate detection of
functions, use has_function with a header in the prefix. This fixes
posix_memalign detection on Android and on MinGW32, MSYS-MinGW-w64, and
old versions of MSYS2-MinGW-w64.

Using the header in the `prefix:` is generally a good idea because of
how macOS does targetting of specific macOS releases at compile time.

This also allows cross-files to override the result by setting
`has_function_stpcpy = false`, etc in [extra properties]

https://bugzilla.gnome.org/show_bug.cgi?id=795876

meson.build

@@ -468,33 +468,13 @@ foreach f : functions
   endif
 endforeach
 
-if cc.get_id() == 'gcc' or cc.get_id() == 'clang'
+# Check that stpcpy() is usable; must use header
-    no_builtin_args = cc.get_supported_arguments(['-fno-builtin'])
+if cc.has_function('stpcpy', prefix : '#include <string.h>')
-else
-    no_builtin_args = []
-endif
-
-# Check that stpcpy() is not a builtin
-if cc.links('''#include <string.h>
-               int main (int argc, char ** argv) {
-                 char p[10];
-                 return stpcpy (p, argv[0]) != NULL;
-               }
-            ''',
-            args : no_builtin_args,
-            name : 'stpcpy() is not a builtin')
   glib_conf.set('HAVE_STPCPY', 1)
 endif
 
-# Check that posix_memalign() is not a builtin
+# Check that posix_memalign() is usable; must use header
-if cc.links('''#include <stdlib.h>
+if cc.has_function('posix_memalign', prefix : '#include <stdlib.h>')
-               int main (int argc, char ** argv) {
-                 void *p;
-                 return posix_memalign (&p, 16, argc);
-               }
-            ''',
-            args : no_builtin_args,
-            name : 'posix_memalign() is not a builtin')
   glib_conf.set('HAVE_POSIX_MEMALIGN', 1)
 endif