[CMake] howto retrigger check_function_exists

[J Decker]

I was trying to add some libraries for CMAKE_REQUIRED_LIBRARIES to be able
to find more symbols, but changing that didn't cause the
check_function-0exists to re-run; is there a shorter way to get it to
re-run without deleting CMakeCache.txt?
I'm not abject to manually editing files; I had tried to remove just the
cached variable reference, but that didn't help.

Same for check_type_size() I guess
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