Add check for CODEQL_EXTRACTOR_CPP_ROOT environment variable

Author: nunoplopes

CMakeLists.txt

@@ -34,7 +34,7 @@ list(APPEND CMAKE_MODULE_PATH "${CLANG_CMAKE_DIR}")
 include(AddLLVM)
 include(AddClang)
 
-if (NOT CMAKE_CXX_COMPILER MATCHES "clang")
+if (NOT CMAKE_CXX_COMPILER MATCHES "clang" AND NOT DEFINED ENV{CODEQL_EXTRACTOR_CPP_ROOT)
   set(CMAKE_CXX_COMPILER "${CLANG_CMAKE_DIR}/../../../bin/clang++")
 endif()