cmake: use ENABLE_SHARED instead of BUILD_SHARED_LIBS

cmake: use ENABLE_SHARED instead of BUILD_SHARED_LIBS

we decided to drop the static libraries from the *-dev debian packages.
so no more tricky changes for supporting creating both libraries.

* partially revert 1dbfb26: s/BUILD_SHARED_LIBS/ENABLE_SHARED/
 - add_library() command use BUILD_SHARED_LIBS as the default library
   type, and all intermediate libraries should static ones. it would
   cause unnecessary confusion if some developer uses add_library() without
   specifying the library type, and ends up with a .so. so we use our
   own setting variables for specifying the type of user facing libraries.
* revert ac47440
 - the BUILD_SHARED_LIBS setting is also populated to the gmock libraries,
   which should be compiled as static ones. otherwise the unit tests will
   crash on exit.

Signed-off-by: Kefu Chai <kchai@redhat.com>