Define ENABLE_CLANG_EXAMPLES instead of relying on BUILD_EXAMPLES

This is a further build fix attempt for r198747 on some Makefile builders where
the value wasn't set at all.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198764 91177308-0d34-0410-b5e6-96231b3b80d8