Add --with-system-pcre; allow building against system PCRE library

src/RageUtil.cpp

@@ -1342,7 +1342,11 @@ bool GetFileContents( const RString &sFile, vector<RString> &asOut )
 	return true;
 }
 
+#ifndef USE_SYSTEM_PCRE
 #include "../extern/pcre/pcre.h"
+#else
+#include <pcre.h>
+#endif
 void Regex::Compile()
 {
 	const char *error;