[CMake] Fixing clang standalone build

I broke this in r288770.

llvm-svn: 288829

llvm/CMakeLists.txt

@@ -803,8 +803,6 @@ endif()
 include(AddLLVM)
 include(TableGen)
 
-set(LLVM_TABLEGEN_FLAGS -I ${LLVM_MAIN_INCLUDE_DIR})
-
 if( MINGW )
   # People report that -O3 is unreliable on MinGW. The traditional
   # build also uses -O2 for that reason:

llvm/cmake/modules/TableGen.cmake

@@ -4,6 +4,10 @@
 
 include(LLVMExternalProjectUtils)
 
+if(LLVM_MAIN_INCLUDE_DIR)
+  set(LLVM_TABLEGEN_FLAGS -I ${LLVM_MAIN_INCLUDE_DIR})
+endif()
+
 function(tablegen project ofn)
   # Validate calling context.
   if(NOT ${project}_TABLEGEN_EXE)