Mark global functions and/or variables in m4(1) static where possible.

This allows compilers and static analyzers to more thorough analysis.

usr.bin/m4/gnum4.c

@@ -60,7 +60,7 @@ int mimic_gnu = 0;
  * Then M4PATH env variable
  */
 
-struct path_entry {
+static struct path_entry {
 	char *name;
 	struct path_entry *next;
 } *first, *last;

usr.bin/m4/main.c

@@ -99,7 +99,7 @@ char scommt[MAXCCHARS+1] = {SCOMMT};	/* start character for comment */
 char ecommt[MAXCCHARS+1] = {ECOMMT};	/* end character for comment   */
 int synccpp;			/* Line synchronisation for C preprocessor */
 
-struct keyblk keywrds[] = {	/* m4 keywords to be installed */
+static const struct keyblk keywrds[] = { /* m4 keywords to be installed */
 	{ "include",      INCLTYPE },
 	{ "sinclude",     SINCTYPE },
 	{ "define",       DEFITYPE },