Check stpcpy() and posix_memalign() to *not* to be built-in

GCC has built-ins for these functions, which might give a compile-only
test an impression that the functions are actually present in the C runtime.
Use a linked test to be sure.

Specifically, both functions are missing on Windows.

https://bugzilla.gnome.org/show_bug.cgi?id=794555

meson.build

@@ -298,7 +298,6 @@ endif
 functions = [
   'alloca',
   'mmap',
-  'posix_memalign',
   'memalign',
   'valloc',
   'fsync',
@@ -310,7 +309,6 @@ functions = [
   'lstat',
   'strsignal',
   'vsnprintf',
-  'stpcpy',
   'poll',
   'vasprintf',
   'setenv',
@@ -401,6 +399,24 @@ foreach f : functions
   endif
 endforeach
 
+# 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;
+               }''', name : 'stpcpy() is not a builtin')
+  glib_conf.set('HAVE_STPCPY', 1)
+endif
+
+# Check that posix_memalign() is not a builtin
+if cc.links('''#include <stdlib.h>
+               int main (int argc, char ** argv) {
+                 void *p;
+                 return posix_memalign (&p, 16, argc);
+               }''', name : 'posix_memalign() is not a builtin')
+  glib_conf.set('HAVE_POSIX_MEMALIGN', 1)
+endif
+
 # Check whether strerror_r returns char *
 if have_func_strerror_r
   if cc.compiles('''#define _GNU_SOURCE