fio Makefile improvement - don't override $(CC)

I tried to build fio 2.0.8 today and found that the Makefile
unconditionally sets CC.
Since I've only got clang on my FreeBSD box it needs to avoid overriding
the default.

Use the default system compiler - don't override $(CC) if already set.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
1 file changed