Update all problematic include guards.

All-caps identifiers that start with an underscore may be reserved
for compiler or library use, and recent versions of gcc or glibc
have been stealing symbols away from us.

sc2/src/config_vc6.h

@@ -7,8 +7,8 @@
  * config_win.h is generated from src/config_win.h.in.
  */
 
-#ifndef _CONFIG_VC6_H
-#define _CONFIG_VC6_H
+#ifndef CONFIG_VC6_H_
+#define CONFIG_VC6_H_
 
 /* Directory where the UQM game data is located */
 #define CONTENTDIR "../content/"
@@ -57,5 +57,5 @@
 /* Defined if your system has wint_t of its own */
 #define HAVE_WINT_T
 
-#endif /* _CONFIG_VC6_H */
+#endif /* CONFIG_VC6_H_ */