CMake: Change libprocess to a shared library.
CMake: Change libprocess to a shared library.

In order to load modules that are themselves based on libprocess,
we must link libprocess as a shared library.  Since modules are
only supported on non-Windows platforms, this changes the default
linking mode to SHARED on non-Windows.

This review replaces: https://reviews.apache.org/r/49924/

Review: https://reviews.apache.org/r/53759/

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