Explicitly set implied defaults for source version

CMake's VERSION_EQUAL considers 1.1 and 1.1.0 to be different
versions. To the _GIT_VERSION patch and tweak variables are set to
their default 0 when absent from the tag if same project component
version is defined.

Change-Id: I9914301413e5e2afcbd864f8ac1208a6c4682031
BUG: Post version is number of commits since tag

The number of commits since the version file changed was being
used. This is not corrected to number of commits since the tag.

Change-Id: Ic2c813f2ec91a96709408a0c375c2865c4c349f8
Merge topic 'SIMPLEITK-327_AddGitBasedVersionInformation' into next

6a0f487 BUG: Post version is number of commits since tag
b753ec7 Explicitly set implied defaults for source version