configure: Define DEFAULT_M4 by searching PATH.

        * configure: Define DEFAULT_M4 by searching PATH.
        * Makefile.in: Use DEFAULT_M4.
Forgot to commit Makefile.in for this change.

From-SVN: r19223

Makefile.in

@@ -100,6 +100,7 @@ GZIPPROG = gzip
 # These values are substituted by configure.
 DEFAULT_YACC = yacc
 DEFAULT_LEX = lex
+DEFAULT_M4 = m4
 
 BISON = `if [ -f $$r/bison/bison ] ; then \
 	    echo $$r/bison/bison -L $$s/bison/ ; \
@@ -121,7 +122,7 @@ LEX = `if [ -f $$r/flex/flex ] ; \
 
 M4 = `if [ -f $$r/m4/m4 ] ; \
 	then echo $$r/m4/m4 ; \
-	else echo m4 ; fi`
+	else echo ${DEFAULT_M4} ; fi`
 
 MAKEINFO = `if [ -f $$r/texinfo/makeinfo/Makefile ] ; \
 	then echo $$r/texinfo/makeinfo/makeinfo ; \