Preprocessor changes in GCC 5 cause guile-snarf to output garbage.

https://bugs.gentoo.org/560736

--- a/libguile/guile-snarf.in
+++ b/libguile/guile-snarf.in
@@ -50,7 +50,7 @@
     ## Apparently, AIX's preprocessor is unhappy if you try to #include an
     ## empty file.
     echo "/* cpp arguments: $@ */" ;
-    ${cpp} -DSCM_MAGIC_SNARF_INITS -DSCM_MAGIC_SNARFER "$@" > ${temp} && cpp_ok_p=true
+    ${cpp} -P -DSCM_MAGIC_SNARF_INITS -DSCM_MAGIC_SNARFER "$@" > ${temp} && cpp_ok_p=true
     grep "^ *\^ *\^" ${temp} | sed -e "s/^ *\^ *\^//" -e "s/\^\ *:\ *\^.*/;/"
 }