Compiler options selected in the main Slicer4
Superbuild harness needs to be propogated to
each of the subsequent External_project builds.

This was tested by setting
export CC=/dev/null
export CXX=/dev/null
cmake -DCMAKE_C_COMPILER:PATH=/usr/bin/gcc -DCMAKE_CXX_COMPILER:PATH=/usr/bin/g++
and then compiling all of Slicer.

Propogate the necessary compiler flags to each
external package consistently.  Additionally,
only propogate the flags that are explicitly needed
for that external project.  In many cases, only the
C compiler options need to be propogated.

Slicer compiler options needed by Extensions as well.
The extension need to have the same compiler used
as the primary Slicer build.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@20464 3bd1e089-480b-0410-8dfb-8563597acbee