upstream's CMake feature detection expects the gtk3 package to supply
not only `cflags` but also `cflags_other`. i think the latter was
intended to be optional, so patch it out since our gtk doesn't supply it.
<1501ad367d/cmake/FindGTK3.cmake (L14-L15)>
the `LIBRE*` and `USE_*` make flags were removed during the cmake
transition in v2.11.0: <https://github.com/baresip/baresip/pull/2354/files>
upstream now detects features on a best-effort basis, with pkg-config.
