libz linked statically with GMX_BUILD_MDRUN_ONLY=ON

Under Linux, the default linking behavior of libz has changed between 2021-beta1 and -beta2.

beta1 and earlier versions: -DGMX_EXTERNAL_ZLIB=ON and -DGMX_EXTERNAL_ZLIB=ON -DGMX_BUILD_MDRUN_ONLY=ON both link to the shared library libz.so.

beta2: -DGMX_EXTERNAL_ZLIB=ON links to the shared library, but -DGMX_EXTERNAL_ZLIB=ON -DGMX_BUILD_MDRUN_ONLY=ON links statically to libz.a.

This can be worked around with an additional manual setting of GMX_PREFER_STATIC_LIBS=OFF, but it's a bit of a change in behavior, and the older behavior (shared linking to system libraries when possible) remains documented as the default in INSTALL under "Static linking".