build: add FALCO_Coverage CMake option
build: add FALCO_Coverage CMake option

With cmake FALCO_Coverage=on the --coverage option
is passed to both clang and gcc to help analyze untested
portions of the code base.  It produces gcov files.

These files can be analyzed by many tools such as lcov,
gcovr, etc.

Here is an example of one such tool, lcov:

 lcov --directory . --capture --output-file coverage.info
 lcov --extract coverage.info '/source/*' --output-file coverage.info
 genhtml coverage.info

Signed-off-by: Chris Goller <ca2248b034ea873fc4f24500b8a0a0828575d45e@gmail.com>

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