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/port.h

@@ -1,5 +1,5 @@
-#ifndef _PORT_H
-#define _PORT_H
+#ifndef PORT_H_
+#define PORT_H_
 
 #include "config.h"
 
@@ -545,5 +545,5 @@ typedef unsigned int wint_t;
 // REJECT_DRIVE_PATH_WITHOUT_SLASH can also be defined, if paths of the form
 // "d:foo/bar" (without a slash after the drive letter) are to be rejected.
 
-#endif  /* _PORT_H */
+#endif  /* PORT_H_ */