Use recommended case for variable names. i.e. matching name of the
module as passed to `find_package`.
For backwards compatibility, the upper case versions of both input and
output variables are used and defined when appropriate. Skip this for
the _FOUND variable because FPHSA already does it. Skip this for the
_VERSION variable because that was recently added and never available
with the old name in a release of CMake.
Check found libraries version to match user required version.
Protobuf compiler executable version is checked to be aligned with found
libraries, raising a warning message otherwise.
Antonio Perez Barrero