find_library: Allow custom lib suffix be used as find path

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.