CMake: use VAR instead of "${VAR}" in if() statements.

Caused annoying warnings on CMake 3.1 (gone in 3.2).

src/CMakeLists.txt

@@ -26,7 +26,7 @@
 # On 4.8.2 strict aliasing causes failure of DebugToolsCylinderRendererTest
 # without any warning, only in release build, any attempt to add debug print
 # results in issue disappearing. Not an issue on Clang or GCC < 4.8.
-if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" AND NOT "${CMAKE_CXX_COMPILER_VERSION}" VERSION_LESS "4.8.0")
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.8.0")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-strict-aliasing")
 endif()
 

src/MagnumExternal/OpenGL/GL/CMakeLists.txt

@@ -26,7 +26,7 @@
 # flextGLPlatform.cpp is compiled as part of Magnum*Context libraries in Platform
 add_library(MagnumFlextGLObjects OBJECT flextGL.cpp)
 
-if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR "${CMAKE_CXX_COMPILER_ID}" MATCHES "(Apple)?Clang")
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?Clang")
     set_target_properties(MagnumFlextGLObjects PROPERTIES COMPILE_FLAGS "-fvisibility=hidden -DFlextGL_EXPORTS")
 else()
     set_target_properties(MagnumFlextGLObjects PROPERTIES COMPILE_FLAGS "-DFlextGL_EXPORTS")