Add workaround for missing 'restrict' keyword.

Add a cpp #define that removes 'restrict' keyword usage unless the
compiler definitely supports C99.  As written, 'restrict' is only
enabled if the compiler supports the -std=gnu99 option (e.g. gcc and
llvm).

Reported by Tobias Hieta.

include/jemalloc/internal/jemalloc_internal_macros.h

@@ -45,3 +45,7 @@
 #ifndef __DECONST
 #  define	__DECONST(type, var)	((type)(uintptr_t)(const void *)(var))
 #endif
+
+#ifndef JEMALLOC_HAS_RESTRICT
+#  define restrict
+#endif