Currently we use ExternalProject_Add to build libunwind subproject. The
usecases it supports are not quite aligned with our. Configuration and
build steps are rerun in case download step sees updated sources. But
we use git submodules to fetch third-party projects and thus subproject
is not rebuild when it's files are changed on disk. Either because of
we doing some sort of experiments locally or because the new sources
are brought by git pull.

Actually libunwind has a CMakeList.txt so that we could try to build it
just as project subdirectory. But it requires quite a fresh CMake
version, currently supports only build on Visual Studio and probably is
not up to date given it's package version variables. Keeping
CMakeLists.txt in sync with main autotools build is additional
maintenance burden. So I'd like to use main autotools build.

So ext_project_autotools function is added to provide nicer build
integration with third-party projects which are build using autotools.
Actually incremental rebuild works only if CMake is at least 3.12 but
I think this should be true for developer installations.

Hopefully we can reuse the function in case we need to bundle more
subprojects with autotools builds.

Follow-up #5665

NO_DOC=build improvement
NO_TEST=build improvement
NO_CHANGELOG=build improvement