cmake: build bundled libyaml as external project

Currently, we simply include the libyaml source directory into the main
project. The problem is that libyaml uses ICU. If ICU is built outside
the main project cmake config, as it's the case with the static-build,
both the main project and libyaml cmake configs will use the same ICU
version. However, if we build ICU in the main project, as we intend to
do to resolve #9242, it may not work. To fix that, we need to use the
external project API to build libyaml.

Needed for #9242

NO_DOC=build
NO_TEST=build
NO_CHANGELOG=build

(cherry picked from commit 8820f5c9)