Fix warnings for -Wmissing-prototypes

Add -Wmissing-prototypes if compiling with gcc
Static where possible

git-svn-id: https://svn.musicpd.org/mpd/trunk@4657 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/utils.h

@@ -39,7 +39,7 @@ void stripReturnChar(char *string);
 
 void my_usleep(long usec);
 
-int ipv6Supported();
+int ipv6Supported(void);
 
 char *appendToString(char *dest, const char *src);