builtin: create builtins.c properly on old cpp

Encontered this on ancient gcc-2.95.3 environment;
src/builtins.def.in -> src/builtins.def generation emitted
^ $
lines (likely by /* */), which where NOT ignored by
src/mkbuiltins and generating bogus builtins.c.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/mkbuiltins

diff --git a/src/mkbuiltins b/src/mkbuiltins
index f562ae2..f412a41 100644
--- a/src/mkbuiltins
+++ b/src/mkbuiltins
@@ -69,7 +69,7 @@
 #include "builtins.h"
 
 !
-< $builtins sed '/^#/d; /^$/d' > $temp
+< $builtins sed '/^#/d; /^ *$/d' > $temp
 awk '{	printf "int %s(int, char **);\n", $1}' $temp
 echo '
 const struct builtincmd builtincmd[] = {'