common: mem.h add alloca definition

We don't support the autoconf ALLOCA/C_ALLOC fallback. If one day,
someone cares for a weird platform, he can fix it.

common/mem.h

@@ -22,10 +22,40 @@
 #include <stdlib.h>
 #include <spice/macros.h>
 
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
+#ifdef STDC_HEADERS
+# include <stdlib.h>
+# include <stddef.h>
+#else
+# ifdef HAVE_STDLIB_H
+#  include <stdlib.h>
+# endif
+#endif
+#ifdef 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
+# ifndef HAVE_ALLOCA
+#  ifdef  __cplusplus
+extern "C"
+#  endif
+void *alloca (size_t);
+# endif
+#endif
+
 typedef struct SpiceChunk {
     uint8_t *data;
     uint32_t len;