package/zlib-ng: drop unrecognized CC variable

Drop unneeded CC variable which is not recognized by cmake:

CMake Warning:
  Manually-specified variables were not used by the project:

    BUILD_DOC
    BUILD_DOCS
    BUILD_EXAMPLE
    BUILD_EXAMPLES
    BUILD_TEST
    BUILD_TESTING
    BUILD_TESTS
    CC

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>