NetHack's regex function prototypes into nhregex.h

There was an issue with Windows mingw build because the function
prototypes were not available. Place them into a distinct
header file nhregex.h and include it from extern.h, and
available for cppregex.cpp to include without the rest of
extern.h (which can give some problems with c++).

sys/share/cppregex.cpp

@@ -10,7 +10,7 @@
 extern "C" {
 #include "config.h"
 #define CPPREGEX_C
-//#include "extern.h"
+#include "nhregex.h"
 } // extern "C"