COMP: Ensure EMSegment is build with default option and available in nightly package.

This commit ensures that "Slicer_BUILD_EMSegment" is properly set after
the first CMake configuration

Following r21598, the option "Slicer_BUILD_BRAINSTOOLS" has been moved after
declaring the option "Slicer_BUILD_EMSegment". Since option "Slicer_BUILD_EMSegment"
is depending on the value of Slicer_BUILD_BRAINSTOOLS, Slicer_BUILD_QTLOADABLEMODULES and
Slicer_USE_PYTHONQT, EMSegment wasn't build by default.

Thanks to Ron Kikinis for reporting the issue.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21612 3bd1e089-480b-0410-8dfb-8563597acbee