Use AC_FUNC_ALLOC

The preferred all-inclusive method for finding alloca().

Reported-by: Richard Palo <richard@netbsd.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89762
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

configure.ac

@@ -416,7 +416,8 @@ AC_ARG_ENABLE(sna,
 	      [SNA="$enableval"],
 	      [SNA=auto])
 
-AC_CHECK_HEADERS([dev/wscons/wsconsio.h alloca.h])
+AC_CHECK_HEADERS([dev/wscons/wsconsio.h])
+AC_FUNC_ALLOCA
 AC_HEADER_MAJOR
 
 if test "x$SNA" != "xno"; then

libobj/alloca.c

@@ -0,0 +1,4 @@
+void *alloca(size_t sz)
+{
+	return NULL;
+}

src/sna/sna_display.c

@@ -42,6 +42,15 @@
 
 #if HAVE_ALLOCA_H
 #include <alloca.h>
+#elif defined __GNUC__
+#define alloca __builtin_alloca
+#elif defined _AIX
+#define alloca __alloca
+#elif defined _MSC_VER
+#include <malloc.h>
+#define alloca _alloca
+#else
+void *alloca(size_t);
 #endif
 
 #include "sna.h"