BUG: Update extension buildsystem to handle source dir without vcs

[jcfr]

This commit prevents extension generated using the wizard but not yet
associated with a source control system from failing to build with a
message like:

  -- Could NOT find Subversion (missing:  Subversion_SVN_EXECUTABLE)
  CMake Error at /path/to/Slicer/Extensions/CMake/SlicerBlockUploadExtension.cmake:38 (message):
    Variable Subversion_SVN_EXECUTABLE is set to an inexistent directory or
    file ! []
  Call Stack (most recent call first):
    /path/to/Slicer/Extensions/CMake/SlicerEnableExtensionTesting.cmake:23 (include)
    /path/to/Slicer-Release/Slicer-build/UseSlicer.cmake:421 (include)
    CMakeLists.txt:21 (include)

If an extension named Foo is not yet associated with a repository is
built, the following author warning is now reported:

  -- Configuring Scripted module: bar
  CMake Warning (dev) at /path/to/Slicer/CMake/SlicerMacroExtractRepositoryInfo.cmake:94 (message):
    Skipping repository info extraction: directory [/path/to/Foo] is not a GIT, SVN
    or CVS checkout
  Call Stack (most recent call first):
    /path/to/Slicer/CMake/SlicerExtensionCPack.cmake:55 (SlicerMacroExtractRepositoryInfo)
    CMakeLists.txt:45 (include)
  This warning is for project developers.  Use -Wno-dev to suppress it.

  -- Configuring done
  -- Generating done
  -- Build files have been written to: /path/to/Foo-build/inner-build

Reported-by: Matt McCormick [email protected]

[jcfr]

Thanks for the review. Integrated in r25984