BUG: Extensions: Do not add CMAKE_PROJECT_NAME to the cache

Following r23180, check for ConfigurePrerequisites component is done
by testing if CMAKE_PROJECT_NAME variable is defined.

Considering that (1) a project statement is expected to exist in the
extension CMakeLists.txt  and (2) the check for ConfigurePrerequisites
component relies on the variable not being set when first configuring,
this commit updates the build system so that the variable is not added
to the cache.

This build system has been tested on Windows and Linux with
Slicer_UPLOAD_EXTENSIONS set to ON (the default).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23181 3bd1e089-480b-0410-8dfb-8563597acbee