cmake: make BUILD_SHARED_LIBS a visible option

It could only be specified on the commandline, and wouldn't show
up as an option in the GUI or when invoked via `cmake -i` at all.

This also tells CMake that it's a BOOL, rather than "UNINITIALIZED".



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149506 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed