https://bugs.gentoo.org/546742
https://bugs.freedesktop.org/show_bug.cgi?id=90615

fix building w/gcc-5

--- man/Makefile.in
+++ man/Makefile.in
@@ -9,7 +9,7 @@
 
 AM_CPPFLAGS = -I$(top_builddir) -I$(top_srcdir)
 filenames.sed: filenames.sed.c
-	$(AM_V_GEN)$(CPP) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) \
+	$(AM_V_GEN)$(CPP) -P $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) \
 	    $(AM_CPPFLAGS) $(CPPFLAGS) $(srcdir)/filenames.sed.c | \
 	    $(SED) -n -e '/s|__/ p' -e '/^\/__/ p' > $@