CMake: Split `CMAKE_FORWARD_ARGS` into `C` and `CXX` versions.
CMake: Split `CMAKE_FORWARD_ARGS` into `C` and `CXX` versions.

By passing `CMAKE_C_FORWARD_ARGS` to C libraries, e.g. curl, libapr,
libevent, and zlib, we avoid CMake warning us about set but unused
variables (since they do not use the `CXX` variables).

Review: https://reviews.apache.org/r/66012

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