These options allow to pass flags that will be appended to the ones
internally determined.

The variable CMAKE_{C,CXX}_FLAGS will then be set using:
 - the list of internal flags: Slicer_{C,CXX}_FLAGS
 - the additional flags passed at configuration time: ADDITIONAL_{C,CXX}_FLAGS

git-svn-id: http://svn.slicer.org/Slicer4/trunk@15175 3bd1e089-480b-0410-8dfb-8563597acbee