config: allow to build tarball when yacc is missing

Autoconf does not handle this scenario as well as Automake does.
If xkbparse.c is already there, do not abort the configuration
if yacc is missing as it is not needed.

If both xkbparse.c and yacc are missing, abort configuration.
Yacc is a required tool to build the package.

Acked-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>

configure.ac

@@ -35,8 +35,14 @@ AM_MAINTAINER_MODE
 # Initialize libtool
 AC_PROG_LIBTOOL
 
-AC_PROG_YACC
 AC_PROG_LEX
+AC_PROG_YACC
+AC_PATH_PROG([YACC_INST], $YACC)
+if test ! -f "src/xkbcomp/xkbparse.c"; then
+   if test -z "$YACC_INST"; then
+      AC_MSG_ERROR([yacc not found - unable to compile src/xkbcomp/xkbparse.y])
+   fi
+fi
 
 m4_ifndef([PKG_PROG_PKG_CONFIG],
     [m4_fatal([Could not locate the pkg-config autoconf macros.