For consistency, always use the M4 detected by configure
(https://trac.macports.org/changeset/137299).

Index: Makefile.in
===================================================================
--- Makefile.in.orig
+++ Makefile.in
@@ -725,6 +725,7 @@
 	-e 's|@LN_S\@|$(LN_S)|g' \
 	-e "s|@ltdl_ac_aux_dir\@|$(ltdl_ac_aux_dir)|g" \
 	-e 's|@macro_dir\@|$(macro_dir)|g' \
+	-e 's|@M4\@|$(M4)|g' \
 	-e 's|@pkgauxdir\@|$(pkgauxdir)|g' \
 	-e 's|@pkgaux_files\@|$(pkgaux_scripts) $(pkgaux_data_files)|g' \
 	-e 's|@pkgaux_install_files\@|$(pkgaux_install_files)|g' \

Index: configure
===================================================================
--- configure.orig
+++ configure
@@ -30301,6 +30301,7 @@
 LD='`$ECHO "$LD" | $SED "$delay_single_quote_subst"`'
 NM='`$ECHO "$NM" | $SED "$delay_single_quote_subst"`'
 LN_S='`$ECHO "$LN_S" | $SED "$delay_single_quote_subst"`'
+M4='`$ECHO "$M4" | $SED "$delay_single_quote_subst"`'
 max_cmd_len='`$ECHO "$max_cmd_len" | $SED "$delay_single_quote_subst"`'
 ac_objext='`$ECHO "$ac_objext" | $SED "$delay_single_quote_subst"`'
 exeext='`$ECHO "$exeext" | $SED "$delay_single_quote_subst"`'
@@ -30733,6 +30734,7 @@
 LD \
 NM \
 LN_S \
+M4 \
 lt_SP2NL \
 lt_NL2SP \
 reload_flag \
@@ -31963,6 +31965,9 @@
 # Whether we need soft or hard links.
 LN_S=$lt_LN_S
 
+# GNU M4.
+M4=$lt_M4
+
 # What is the maximum length of a command?
 max_cmd_len=$max_cmd_len
 
 
Index: libtoolize.in
===================================================================
--- libtoolize.in.orig
+++ libtoolize.in
@@ -44,6 +44,7 @@
 : ${FGREP="@FGREP@"}
 : ${GREP="@GREP@"}
 : ${LN_S="@LN_S@"}
+: ${M4="@M4@"}
 : ${SED="@SED@"}