[CMake] Fix set_output_directory to do nothing if *_OUTPUT_INTDIR was not given.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212313 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] Introduce LLVM_SHLIB_OUTPUT_INTDIR.

For now, its user is configure_lit_site_cfg().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212314 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] Introduce LLVM_ENABLE_PLUGINS as an internal option. BUILD_SHARED_LIBS may not control enable/disable plugins.

FIXME: Make this configurable.

FIXME: "ENABLE_SHARED" doesn't make sense, since it is used just for plugins. We may rename it.
I introduced config.enable_shared in r120273.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212315 91177308-0d34-0410-b5e6-96231b3b80d8