CMakeLists.txt: fix version in zlib.pc when building statically

CMakeLists.txt: fix version in zlib.pc when building statically

When building statically (i.e. with BUILD_SHARED_LIBS=OFF),
ZLIB_FULL_VERSION is not set resulting in an empty version in zlib.pc
and the following build failure with transmission:

checking for ZLIB... configure: error: Package requirements (zlib >= 1.2.3) were not met:

Package dependency requirement 'zlib >= 1.2.3' could not be satisfied.
Package 'zlib' has version '', required version is '>= 1.2.3'

Fixes:
 - http://autobuild.buildroot.org/results/b3b882482f517726e5c780ba4c37818bd379df82

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>