Add rules for cmake 3.11 to not error

As of cmake 3.11, the add_library target must have sources
associated with it. If there are sources, just compile
the file empty.cpp.

Change-Id: I7fec03dc0f0d2e6098572ed6e9fcdfaac3811024
Reviewed-on: https://skia-review.googlesource.com/129189
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed