Add ability to use a local copy of bacnet-stack

[abelino]

To use a local copy of bacnet-stack w/ relevant patches already applied, run:

ERL_EI_LIBDIR=~/.config/asdf/installs/erlang/27.2.4/usr/lib \
ERL_EI_INCLUDE_DIR=~/.config/asdf/installs/erlang/27.2.4/usr/include \
cmake \
  -B_build/external \
  -DCPM_bacnet_SOURCE=/tmp/bacnet-stack

cd _build/external
make

Replace ERL_EI_LIBDIR with the correct path to libei.a.
Replace ERL_EI_INCLUDE_DIR with the correct path to libei includes.
Replace CPM_bacnet_SOURCE with the path to your locally patched bacnet-stack repo.