Only necessary when other patches touch flex.skl.

--- origsrc/flex-2.5.37/Makefile.am	2012-08-02 12:14:45.000000000 -0500
+++ src/flex-2.5.37/Makefile.am	2013-04-18 22:53:08.033219500 -0500
@@ -129,7 +129,7 @@ AM_CPPFLAGS = -DLOCALEDIR=\"$(localedir)
 LIBS = @LIBINTL@ @LIBS@
 
 skel.c: flex.skl mkskel.sh flexint.h tables_shared.h
-	sed 's/m4_/m4postproc_/g; s/m4preproc_/m4_/g' $(srcdir)/flex.skl | $(m4) -P -DFLEX_MAJOR_VERSION=`echo $(VERSION)|cut -f 1 -d .` -DFLEX_MINOR_VERSION=`echo $(VERSION)|cut -f 2 -d .` -DFLEX_SUBMINOR_VERSION=`echo $(VERSION)|cut -f 3 -d .` | sed 's/m4postproc_/m4_/g' | $(SHELL) $(srcdir)/mkskel.sh  >skel.c
+	sed 's/m4_/m4postproc_/g; s/m4preproc_/m4_/g' $(srcdir)/flex.skl | $(m4) -I $(srcdir) -P -DFLEX_MAJOR_VERSION=`echo $(VERSION)|cut -f 1 -d .` -DFLEX_MINOR_VERSION=`echo $(VERSION)|cut -f 2 -d .` -DFLEX_SUBMINOR_VERSION=`echo $(VERSION)|cut -f 3 -d .` | sed 's/m4postproc_/m4_/g' | $(SHELL) $(srcdir)/mkskel.sh  >skel.c
 
 # Explicitly describe dependencies.
 # You can recreate this with `gcc -I. -MM *.c'