cmake : enable building llama.cpp using system libggml (llama/12321)

cmake : enable building llama.cpp using system libggml (llama/12321)

* cmake: Factor out compiler flag function from ggml

llama.cpps's build requires it, too, and we may want to make use of it
without add_subdirectory(ggml).

* cmake: Enable building against system ggml

This facilitates package maintenance for Linux distributions, where the
libggml library most likely will be shipped as an individual package
upon which a llama.cpp package depends.