available targets unless LLVM_INCLUDE_X is ON. LLVM_BUILD_X implies
LLVM_INCLUDE_X"

It breaks the configuration phase when cmake is invoked without
parameters, it is too complex for the purpose and introduces an
incovenience for the user (as both LLVM_BUILD_X and LLVM_INCLUDE_X
must set to OFF for not including X on the build)

llvm-svn: 114795