The old *_VERSION_STRING is silently left set for backward compat.

OS Elbrus 6.0-rc1 to rc3 have hg executable broken
because of python2 and python3 module directories conflict.
Here, we avoid hg related tests if such case is detected.

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.

This follows the approach from commit a7b09e7f43 (FindProtobuf: Rename
variables to match case of module name, 2016-03-01, v3.6.0-rc1~273^2).

Issue: #20370

cda401fe Tests: Simplify CMakeOnly.AllFindModules policy settings
1a2ede17 Tests: Fix FindModulesExecuteAll when KDE4 is installed

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !736

Some machines have incomplete or otherwise broken installations of
specific packages.  Allow local configuration to prevent the test from
failing on such packages if the version number cannot be extracted.

a7b09e7f FindProtobuf: Rename variables to match case of module name

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.

Also detect the library version number.  Provide results as variables
and as an imported target, LTTng::UST.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>