Fix warning in the CMake build about redefining LLVM_PATH_XDOT_PY by making sure...

Fix warning in the CMake build about redefining LLVM_PATH_XDOT_PY by making sure both definitions are identical. This used to produce warnings like

In file included from .../llvm/trunk/tools/lto/LTOCodeGenerator.cpp:45:
In file included from .../llvm/trunk/include/llvm/Support/system_error.h:225:
.../include/llvm/Config/config.h:591:9: warning: 'LLVM_PATH_XDOT_PY' macro redefined
#define LLVM_PATH_XDOT_PY ".../bin/xdot.py"
        ^
.../include/llvm/Config/llvm-config.h:98:9: note: previous definition is here
#define LLVM_PATH_XDOT_PY ""
        ^
1 warning generated.

(Paths edited for clarity)

Note: This only affected people who had xdot.py installed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132050 91177308-0d34-0410-b5e6-96231b3b80d8