COMP: Set CMake policy CMP0054 to shut-up associated warning

This commit set CMake policy CMP0054 to shut-up associated warning

This commit will avoid the following warning:

CMake Warning (dev) at /path/to/cmake-3.1/Modules/ExternalProject.cmake:919 (if):
Policy CMP0054 is not set: Only interpret if() arguments as variables or
keywords when unquoted. Run "cmake --help-policy CMP0054" for policy
details. Use the cmake_policy command to set the policy and suppress this
warning.

Quoted variables like "jqPlot_DIR" will no longer be dereferenced
when the policy is set to NEW. Since the policy is not set the OLD
behavior will be used.
Call Stack (most recent call first):
/path/to/cmake-3.1/Modules/ExternalProject.cmake:1764 (_ep_write_initial_cache)
/path/to/cmake-3.1/Modules/ExternalProject.cmake:1997 (_ep_add_configure_command)
SuperBuild.cmake:333 (ExternalProject_Add)
CMakeLists.txt:592 (include)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23904 3bd1e089-480b-0410-8dfb-8563597acbee