Don't use IF_LINT in Bison sources.

It creates unnecessary differences between the sources that Bison
maintainers build and test (given that maintainers normally
configure with --enable-gcc-warnings) and the sources that Bison
users build.  Instead, use PACIFY_CC, which doesn't.  This change
fixes compiler warnings reported by Tys Lefering at
<http://lists.gnu.org/archive/html/bison-patches/2011-05/msg00004.html>.
* configure.ac: Don't AC_DEFINE lint regardless of the configure
options.  This change affects imported gnulib sources, where
IF_LINT still appears and depends on lint.
* src/scan-gram.l, src/scan-skel.l: Replace uses of IF_LINT with
PACIFY_CC.
* src/system.h (IF_LINT): Remove cpp macro.
(PACIFY_CC): New cpp macro.

src/scan-skel.l

@@ -58,7 +58,7 @@ static void fail_for_invalid_at (char const *at);
 %%
 
 %{
-  int out_lineno IF_LINT (= 0);
+  int out_lineno PACIFY_CC (= 0);
   char *outname = NULL;
 
   /* Currently, only the @warn, @complain, @fatal, @warn_at, @complain_at, and