Fixing docs build with gcc-4.5

http://bugs.gentoo.org/317175

Patch scavenged from
http://lists.gnu.org/archive/html/guile-devel/2009-11/msg00078.html

--- libguile/guile-snarf-docs.in
+++ libguile/guile-snarf-docs.in
@@ -23,4 +23,4 @@
 ## Let the user override the preprocessor autoconf found.
 test -n "${CPP+set}" || CPP="@CPP@"
 
-${CPP} -DSCM_MAGIC_SNARF_DOCS "$@"
+${CPP} -P -DSCM_MAGIC_SNARF_DOCS "$@"