Build fix following r198747

Convert CMake CLANG_BUILD_EXAMPLES to a boolean value for consumption by
Python, otherwise the raw config string gets passed through which may not be
0/1.

The if/else matches the technique used to solve the problem in LLVM's own
CMakeLists.txt.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198750 91177308-0d34-0410-b5e6-96231b3b80d8