The XINPUT library is unlikely to be available on many systems,
including a lot of Linux distributions. This will warn developers in
advance by (a) not searching for XINPUT unless it was explicitly
requested, and (b) printing a warning if they do request it.

REVIEW: 125935

While the search is unlikely to succeed on Windows, having different
behaviour between the platforms (eg: find_package(Wayland REQUIRED) was
not fatal on Windows, even though Wayland_FOUND would always be FALSE)
is not ideal, and if someone did port them to Windows for some reason,
the find modules should support that.

If applications actually want different behaviour between platforms
(like requiring a module on Unix, but not on Windows), they should
implement that logic themselves (since they will have to deal with
targets not being defined, etc, anyway).

REVIEW: 120481

ECMUseFindModules allows find modules to be copied to a local directory.
These find modules may use ECMFindModuleHelpers, but they will not be in
the same relative location to ECMFindModuleHelpers.cmake and there is no
guarantee that ECMFindModulesHelpers.cmake will be in the CMake module
path.

To solve this, we make sure there is always a stub file in the same
directory as the find modules that includes the real
ECMFindModuleHelpers.cmake. The one installed with ECM just includes
"../modules/ECMFindModuleHelpers.cmake", while ecm_use_find_modules
generates a stub that uses an absolute path.

REVIEW 117658

The traditional *_LIBRARIES, *_INCLUDE_DIRS and *_DEFINITIONS do have
some uses - they make it easier to create package config files that use
found libraries in their link interface.  So this makes sure these
variables are set by ecm_find_package_handle_library_components() (and
hence by FindWayland.cmake and FindXCB.cmake).

REVIEW: 117585

This is deliberately modelled very closely on CMake's documentation
system.  It's a hefty patch, because it involved changing all the
documentation to be in reStructuredText format.  I also cleaned up the
copyright/license statements at the same time.

Note that the find modules contain the full license, due to the fact
that ecm_use_find_module() copies them out of the ECM distribution.

This currently mostly contains macros for handling components;
FindWayland and FindXCB are ported to use this module, which comes with
various improvements for them.

REVIEW: 116653

 * define imported targets for all components
 * add version information
 * print message on Windows
 * add package properties
 * use handle components

REVIEW: 116046