pngquant: doesn't support building with ccache

pngquant: doesn't support building with ccache

pngquant's homegrown configure script doesn't understand CC with a
space, so we can't use ccache with it. Only the host variant is
affected: for the target, we call ccache from the toolchain-wrapper so
it's transparent to the configure script.

This wasn't seen in the autobuilders since they never enable
BR2_CCACHE.

To test, use any configuration and do
make BR2_CCACHE=y host-pngquant

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>