Fix to compile successfully with -Wundef

-Wundef warns about "#if FOO" when FOO is undefined.

gcc warns about this even for an #if branch that is not reached (due
to being excluded by an earlier #if), hence the checks before using
SYS_PREFIX.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2787
TEST="gcc -c linux_syscall_support.h -Wundef" on gcc 4.4.3 (Ubuntu Lucid)

Review URL: https://chromiumcodereview.appspot.com/10536175

git-svn-id: http://linux-syscall-support.googlecode.com/svn/trunk@11 829466d3-f3f5-3ae4-62ad-de35cf9bba21