BUG: Update extension buildsystem to handle source dir without vcs

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 <matt.mccormick@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25984 3bd1e089-480b-0410-8dfb-8563597acbee