Add a new `CMAKE_FIND_LIBRARY_CUSTOM_LIB_SUFFIX` variable to allow use
of a custom suffix on `lib` directory names. This is a more general
option than that added by commit v3.7.0-rc1~504^2 (Teach find_library
and find_package to search lib32 paths, 2016-06-10). It allows the find
path to be more deterministic on custom setups.
See discussion in #10287 and #15994.