building with -qt-zlib causes runtime failure

Statically compiling the zlib (version 1.2.3) from the Qt repository into QtCore can lead to a crash when system libraries (such as libxml2) that link dynamically against zlib 1.2.4 or newer try to use zlib functions.

The problem is that new functions have been added to zlib in 1.2.4 (gzopen64 etc.) and the new zlib header defines gzopen to gzopen64 when possible. So when libxml2 calls gzopen, it will call gzopen64 from the shared library. Other functions, such as gzread, do not have new variants and are thus resolved to the version from QtCore.

Mixing the new gzopen64 with the old gzread does not work correctly, so gzread will always fail.

Suggestion: Update the zlib in the Qt repository.