redis: fix build with ccache enabled

Some parts of redis fail to build when ccache is enabled, due to one of our
own patches. The construct
    make CC=$(CC) target1 target2
would result in
    make CC=ccache <tuple>-gcc target1 target2

and here <tuple>-gcc would be treated as a target and the compiler used
would be just 'ccache'.

Reported-by: Johan Sagaert <sagaert.johan@skynet.be>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>