diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 55b4d2d..66b4e96 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -3990,7 +3990,7 @@ $(genprog:%=build/gen%$(build_exeext)): build/gen%$(build_exeext): build/gen%.o
 # bconfig.h because AIX requires _LARGE_FILES to be defined before
 # any system header is included.
 gengtype-lex.c : gengtype-lex.l
-	-$(FLEX) $(FLEXFLAGS) -o$@ $< && { \
+	-unset M4; $(FLEX) $(FLEXFLAGS) -o$@ $< && { \
 	  echo '#include "bconfig.h"' > $@.tmp; \
 	  cat $@ >> $@.tmp; \
 	  mv $@.tmp $@; \