Make using Yacc optional, disabled by default

A built-in GNU Make rule causes code/tools/lcc/lburg/gram.y to replace
gram.c if gram.y has a newer modified time. This causes git diff to
pick up changes to gram.c, which seems to have been manually modified
to fix warnings and may vary by Yacc used to create it. It also
requires installing a program to generate a file that already exists
in a usable state in the code repository.

So replace the built-in rule so it is only used if USE_YACC is 1
(defaults to 0). The Yacc executable name can be overriden using
`make YACC=yacc` like before.

I preferred to touch gram.c instead of installing Yacc because of the
problems it causes. It doesn't really seem like a good idea to recommend
others do that instead of disabling Yacc the Makefile though.

README.md

@@ -89,6 +89,7 @@ Makefile.local:
   SERVERBIN            - rename 'ioq3ded' server binary
   CLIENTBIN            - rename 'ioquake3' client binary
   USE_RENDERER_DLOPEN  - build and use the renderer in a library
+  USE_YACC             - use yacc to update code/tools/lcc/lburg/gram.c
   BASEGAME             - rename 'baseq3'
   BASEGAME_CFLAGS      - custom CFLAGS for basegame
   MISSIONPACK          - rename 'missionpack'