Fix alloca detection.

include/libfirm/adt/xmalloc.h

@@ -18,12 +18,14 @@
 #include <string.h>
 
 /* Includes for alloca() */
-#if defined(__GNUC__) && !defined(alloca)
-#define alloca(x)       __builtin_alloca(x)
-#elif defined(__WIN32)
-#include <malloc.h>
-#else
-#error do not know how to get alloca
+#ifndef alloca
+#	if defined(__GNUC__)
+#		define alloca(x)       __builtin_alloca(x)
+#	elif defined(__WIN32)
+#		include <malloc.h>
+#	else
+#		error do not know how to get alloca
+#	endif
 #endif
 
 #include "../begin.h"