I tried to compile with gcc on Tru64, and got the following error.
SDL_systimer.c:45:45: error: operator '&&' has no right operand

It succeeds if changing
#if (defined _POSIX_TIMERS && _POSIX_TIMERS > 0)
to
#if (defined _POSIX_TIMERS && _POSIX_TIMERS + 0 > 0)

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401329