COMP: Fix CMP0054 warning in SlicerMacroGetCompilerName()

Thanks to "Csaba Pinter <csaba.pinter@queensu.ca>" for reporting the issue.

This commit fixes warnings occurring when using CMake >= 3.1.0

This commit fixes the following warning:

CMake Warning (dev) at /path/to/Extensions/CMake/SlicerMacroGetCompilerName.cmake:7 (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 "EXTENSION_COMPILER" 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/Extensions/CMake/SlicerBlockUploadExtensionPrerequisites.cmake:10 (SlicerMacroGetCompilerName)
/path/to/Slicer-build/UseSlicer.cmake:285 (include)
VffFileReader/CMakeLists.txt:18 (include)
This warning is for project developers. Use -Wno-dev to suppress it.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23900 3bd1e089-480b-0410-8dfb-8563597acbee