update CMake Cairo discovery to match PangoCairo

update CMake Cairo discovery to match PangoCairo

This does the equivalent of the previous commit, to now use pkg-config to
discover Cairo on non-Windows platforms and to suppress the CMake warning:

  CMake Warning (dev) at ...FindPackageHandleStandardArgs.cmake:272 (message):
    The package name passed to `find_package_handle_standard_args` (CAIRO) does
    not match the name of the calling package (Cairo).  This can lead to
    problems in calling code that expects `find_package` result variables
    (e.g., `_FOUND`) to follow a certain pattern.
  Call Stack (most recent call first):
    cmake/FindCairo.cmake:12 (find_package_handle_standard_args)
    CMakeLists.txt:81 (find_package)
  This warning is for project developers.  Use -Wno-dev to suppress it.