BUG: Add SimpleITK_VERSION variable defined after find_package

No SimpleITK_VERSION variables were defined after neither
find_package(SimpleITK) nor include(${SimpleITK_USE_FILE}). The
PACKAGE_VERSION variable is now defined in the version config file
which automatically sets the major.minor.tweak component
variables.

Change-Id: I59ff2e500a56e0400655673ec6d64a80cd5d6ba1
Merge topic 'SIMPLEITK-518_AddVersionVariablesForFindPackage' into next

c40dfaf6 BUG: Add SimpleITK_VERSION variable defined after find_package
Require SimpleITK major version to match for compatible version

When a version is specified for find_package, it's required that the
major version match. In general versioning schemes changes is the
major version represent a change in compatibility.

Change-Id: Ib6e6a9315d94ff351746d6420ea59f3e2b91ab39
Merge topic 'SIMPLEITK-518_AddVersionVariablesForFindPackage' into next

c2680aa0 Require SimpleITK major version to match for compatible version