Fix clang on Windows warnings

- STDCALL is ignored for variable argument functions.
- __attribute__ does work for Clang (also if that pretends to be MSVC)
- remove unused function
- simplify ma_getopt, fixes some strange compile error in clang (about
SSE intrinsics)

- fix some clang warnings

unittest/libmariadb/ma_getopt.c

@@ -45,10 +45,9 @@ Cambridge, MA 02139, USA.  */
 #define const
 #endif
 #endif
-
-#include <ma_global.h>				/* Changes for mysys */
-#include <ma_string.h>
-
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
 /* Comment out all this code if we are using the GNU C Library, and are not
    actually compiling the library itself.  This code is part of the GNU C
    Library, but also included in many other GNU distributions.	Compiling