diff --git a/modules/alloca-opt b/modules/alloca-opt
index 30d0ad4e7..afa670ee0 100644
--- a/modules/alloca-opt
+++ b/modules/alloca-opt
@@ -21,12 +21,16 @@ BUILT_SOURCES += $(ALLOCA_H)
 # doesn't have one that works with the given compiler.
 if GL_GENERATE_ALLOCA_H
 alloca.h: alloca.in.h $(top_builddir)/config.status
-	$(AM_V_GEN)rm -f $@-t $@ && \
-	$(MKDIR_P) '%reldir%' && \
-	{ echo '/* DO NOT EDIT! GENERATED AUTOMATICALLY! */'; \
-	  sed -e 's|@''HAVE_ALLOCA_H''@|$(HAVE_ALLOCA_H)|g' < $(srcdir)/alloca.in.h; \
-	} > $@-t && \
-	mv -f $@-t $@
+	$(AM_V_at)rm -f $@-t $@
+	$(AM_V_at)$(MKDIR_P) '%reldir%'
+	$(AM_V_at)sed -n \
+           -e h \
+           -e '1s,.*,/* DO NOT EDIT! GENERATED AUTOMATICALLY! *,w $@-t' \
+           -e g \
+           -e 's|@''HAVE_ALLOCA_H''@|$(HAVE_ALLOCA_H)|g' \
+           -e 'w $@-t' \
+           $(srcdir)/alloca.in.h
+	$(AM_V_GEN)mv -f $@-t $@
 else
 alloca.h: $(top_builddir)/config.status
 	rm -f $@