Allow to use functions from BuildFlatBuffers.cmake from a flatbuffe…

[KerstinKeller]

…rs installation installed with CMake.

Solves #7909

I tried to build and install flatbuffers (including flatc) with CMake.
When trying to integrate that install into my project, I realized that the very convenient flatbuffers_generate_headers functions could not be used.
This PR fixes that.

(Instead of adding this clause

elseif(TARGET flatbuffers::flatc)
    set(FLATC_TARGET flatbuffers::flatc)
    set(FLATC flatbuffers::flatc)

it would also be possible to create an ALIAS target flatbuffers::flatc (e.g. add_executable(flatbuffers::flatc ALIAS flatc) and reference that throughout the the whole project. Doing the same for the other targets (e.g. flatbuffers::flatbuffers... ) allows the same experience for both consumers who build flatbuffers as part of their build, or use an installed version instead. But this PR goes the easier, quick way to fix the described problem.)

[dbaileychess]

Feel free to do another PR with the alias, as that sounds better.