Switch to intmax_t (rather than int) for #if expressions

This is what the C99 specification demands. And the GLSL specification
says that we should follow the "standard C++" rules for #if condition
expressions rather than the GLSL rules, (which only support a 32-bit
integer).
diff --git a/glcpp.h b/glcpp.h
index 33ece8f..503731b 100644
--- a/glcpp.h
+++ b/glcpp.h
@@ -24,6 +24,8 @@
 #ifndef GLCPP_H
 #define GLCPP_H
 
+#include <stdint.h>
+
 #include <talloc.h>
 
 #include "hash_table.h"