We have to give cmake the explicit zlib location as the internal testing
environment has C:/msys64/usr/bin in the PATH and zlib-devel installed.
cmake has heuristics in find_package which sees this /bin suffix in PATH
and adds C:/msys64/usr/ to the find_package search locations. Doing a
find_package(zlib) in this environment makes the find module for it (FindZLIB)
find the header zlib.h in C:/msys64/usr/include while the library will still
be taken from the testinstall location masking the issue in the log. To satisfy
the dependency cmake adds C:/msys64/usr/include as an include directory. This
makes cl.exe build with mixed C and C++ standard lib headers breaking the
build.
This issue was previously masked by cmake writing absolute paths for zlib and
other dependencies into the grpc cmake exports.