COMP: Consistently specify optional external project CMake arguments

Easier maintenance of the build system by using the variable
"EXTERNAL_PROJECT_OPTIONAL_ARGS" to accumulate optional CMake arguments.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21989 3bd1e089-480b-0410-8dfb-8563597acbee
STYLE: Fix indent in SimpleITK and teem external project

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21990 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix CMake warning related to unused CMAKE_BUILD_TYPE variable

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21991 3bd1e089-480b-0410-8dfb-8563597acbee