Clean up GCC warnings (2019-08-29 edition)

Updated most parts of the code that were throwing warnings in GCC.

src/parse.gperf

@@ -37,6 +37,10 @@ struct cfg
 #define NEGBOOL(x) (void*)(offsetof(struct cfginfo,x)|NEG)
 #define IMM(x)     imm_##x
 
+/* Prototypes of what's in tuxpaint.c: */
+void show_version(int details);
+void show_usage(int exitcode);
+
 static void imm_version(void)
 {
   show_version(0);