COMP: Restore CTest driven build of extension

Following commit r22038 [1], UseSlicer.cmake file has been updated to
NOT call "project()". This didn't cause problems when manually
configuring and building the extension. Problems arise when the
extension build is driven using ctest.

More particularly, calling "ctest_build()" [2] on a build tree
configured without an explicit call to "project()" failed on
windows. In that specific case, the command line generated to
drive the build is expecting a solution file named
after <CTEST_PROJECT_NAME>.sln. In our case, only the
file "MyProject.sln" was existing.

This commit ensure that a properly named solution file will be
created by calling "project(${EXTENSION_NAME})" when appropriate.

Thanks to Csaba Pinter for reporting the issue.

Discussion: http://slicer-devel.65872.n3.nabble.com/Extensions-failed-to-build-on-windows-last-night-tt4028763.html

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=22038
[2] https://github.com/Slicer/Slicer/blob/cd985077aec48b3981bf114eba7f707c0eb55299/Extensions/CMake/SlicerBlockBuildPackageAndUploadExtension.cmake#L155

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22050 3bd1e089-480b-0410-8dfb-8563597acbee