Prevent memset from calling itself

With -ftree-loop-distribute-patterns, GCC detects that the body of memset
is equivalent to memset, and so turns it into a call to memset, causing
infinite recursion.

Closes #170.

util/string.c

@@ -4,6 +4,11 @@
 #include <stdint.h>
 #include <ctype.h>
 
+#ifdef __GNUC__
+// Don't let GCC pattern-match these functions' bodies into self-calls
+#pragma GCC optimize ("no-tree-loop-distribute-patterns")
+#endif
+
 void* memcpy(void* dest, const void* src, size_t len)
 {
   const char* s = src;