Fix Compiler id variable name.

Tests/Module/GenerateExportHeader/CMakeLists.txt

@@ -3,7 +3,7 @@ cmake_minimum_required(VERSION 2.8.5 FATAL_ERROR)
 project(GenerateExportHeader)
 
 # Prevent timeout on Watcom by not running the tests.
-if ("${CMAKE_COMPILER_ID}" MATCHES Watcom)
+if ("${CMAKE_CXX_COMPILER_ID}" MATCHES Watcom)
   file(WRITE
     "${CMAKE_CURRENT_BINARY_DIR}/main.cxx"
     "int main() { return 0; }