Fix C++ build failure due to format macros

PRIi64 etc. may not be available without this macro definition, on
pre-C++11 compilers. At least I think that's the issue.

Author: nikic

main/php_stdint.h

@@ -36,6 +36,9 @@
 # ifndef __STDC_CONSTANT_MACROS
 #  define __STDC_CONSTANT_MACROS
 # endif
+# ifndef __STDC_FORMAT_MACROS
+#  define __STDC_FORMAT_MACROS
+# endif
 #endif
 
 #if defined(_MSC_VER)