ccache: Force ccache to use its internal zlib

The problem is that without this, ccache would link against the zlib
of the build system, but we might build and install a different
version of zlib in $(O)/host afterwards, which ccache will pick
up. This might break if there is a version mismatch. A solution would
be to add host-zlib has a dependency of ccache, but it would require
tuning the zlib .mk file to use HOSTCC_NOCCACHE as the
compiler. Instead, we take the easy path: tell ccache to use its
internal copy of zlib, so that ccache has zero dependency besides the
C library.

Fixes bug #4808.

Thanks to Raúl Sánchez Siles <rsanchezs@infoglobal.es> for reporting
the bug and testing the proposed solution.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>