share/doc/usd/{30.rogue,31.trek}: make fails if the games source
tree is not present (see the mail that I have sent yesterday).
This patch makes the Makefiles test if the games source
tree is present. If it is not present, the Makefiles default to
no operation.
[Note: I'm not sure if the Makefile changes might not be collapsable into
a shorter format, but I'll just leave it this way for now and be safe -jkh]
Submitted by: Remy Card <Remy.Card@masi.ibp.fr>
