Remove quoted variable de-referencing as per CMP0054

CMakeLists.txt

@@ -261,7 +261,7 @@ IF ((CMAKE_C_COMPILER_ID MATCHES "GNU") AND NOT CMAKE_COMPILER_IS_MINGW)
   ENDIF()
 ELSEIF(MSVC)
   # enable multi-core compilation with MSVC
-  IF( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" ) # clang-cl
+  IF(CMAKE_CXX_COMPILER_ID MATCHES "Clang" ) # clang-cl
     ADD_COMPILE_OPTIONS(/bigobj /W4 /WX )
   ELSE() # msvc
     ADD_COMPILE_OPTIONS(/MP /bigobj /W4 /WX)
@@ -271,7 +271,7 @@ ELSEIF(MSVC)
     ADD_COMPILE_OPTIONS(/wd4351)
   ENDIF()
   SET(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /D_DEBUG /Zi /Od")
-ELSEIF ( "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
+ELSEIF (CMAKE_CXX_COMPILER_ID MATCHES "Clang" )
   IF(NOT ASSIMP_HUNTER_ENABLED)
     SET(CMAKE_CXX_STANDARD 11)
     SET(CMAKE_POSITION_INDEPENDENT_CODE ON)