[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
2 files changed