When testing CMAKE_<LANG>_COMPILER_ID values, do not explicitly
dereference or quote the variable. We want if() to auto-dereference the
variable and not its value. Also replace MATCHES with STREQUAL where
equivalent.
Add "LinkStatic" test that links a static executable against "libm.a".
Pass both "/usr/lib/libm.a" and "-lm" to target_link_libraries to
trigger the link type logic for both cases. If CMake incorrectly
switches the link type to shared for "-lm" then the link will fail.