This diff improves the UX when a library is missing:

• If the header is not found, there is no need to search for anything

else. find_package_handle_standard_args() is the only useful call to
indicate the failure with a nice error message.

• There is no need to set the version number to some garbage value. If

the version cannot be determined it should just remain unset.

ninja check

Uninstall some library (miniupnpc was used for this test).

cmake -GNinja ..

Check it fails to find the lib and outputs a meaningful error message
(missing header)
Reinstall the library, then

cmake -GNinja .. # Don't nuke the cache !

Check the lib is now found.

ninja check