util.h: also sniff for C99 before trying to use stdbool.h (issue4605)

stdbool.h isn't required until C99. Sigh.

mercurial/util.h

@@ -218,7 +218,7 b' enum normcase_spec {'
 
 #define MIN(a, b) (((a)<(b))?(a):(b))
 /* VC9 doesn't include bool and lacks stdbool.h based on my searching */
-#ifdef _MSC_VER
+#ifdef _MSC_VER || __STDC_VERSION__ < 199901L
 #define true 1
 #define false 0
 typedef unsigned char bool;