Add "ObjectLibrary" test to build and use OBJECT libraries.  Build
multiple object libraries in separate directories with different flags.
Use a custom command to generate a source file in one OBJECT library.

Reference the OBJECT libraries for inclusion in a STATIC library, a
SHARED library, and an EXECUTABLE target.  Use the static and shared
libraries each in executables that end up using the object library
symbols.  Verify that object library symbols are exported from the
shared library.