Fixing build with gcc-4.5

https://bugs.gentoo.org/show_bug.cgi?id=317815

Patch written by Kacper Kowalik <xarthisius.kk@gmail.com>

--- configure
+++ configure
@@ -2636,7 +2636,7 @@
 fi
 
 cat >do_mk_btri.sh <<END_OF_DO_MK_BTRI
-$dcpp "\$1" | sed -n "/^[ 	]*[\\"'%]/p" | $mk_btri_cmd
+$dcpp -P "\$1" | sed -n "/^[ 	]*[\\"'%]/p" | $mk_btri_cmd
 END_OF_DO_MK_BTRI
 
 cat >TABMakefile <<END_OF_TABMAKEFILE1