Enable CMake checks for MSVC

Markus Rickertrequested to merge
rickert/libxml2:cmake-config-checks into master

This commit enables the currently disabled CMake checks for MSVC, e.g., to correctly set HAVE_STDINT_H in config.h. These checks were omitted in earlier versions as MSVC was using a separate win32config.h for creating config.h, which is no longer used since commit 4b6e6828.

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