COMP: Fix unused CMake variable Teem_VTK_TOOLKITS_IPATH warning

Following r21963, Teem_VTK_MANGLES is set to OFF. For that reason,
the variable Teem_VTK_TOOLKITS_IPATH is not required anymore.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21994 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Remove unused Slicer superbuild boolean variable

The variable "Slicer_BUILD_Extensions" originally introduced
by commit r21550 is not used.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21995 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix CMake warning related to WITH_MEMCHECK CMake variable

Commented the variable waiting the build system is updated to support
running of the test using valgrind.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21996 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix unused CMake variable warning related to DOCUMENTATION_* variable

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21997 3bd1e089-480b-0410-8dfb-8563597acbee