cmake: add install target

Allow the built library and its headers to be installed to
CMAKE_INSTALL_PREFIX.
cmake: don't try to resolve libm to a file

check_library_exists() already tells us that libm.so can be found
somewhere in the current search path. It seems rather silly to do the
entire search once more. One that fails on Android, by the way.
Increase the warning level

The original spooles 2.2 makefiles set warning level all by default.
Let's raise that to extra.