On macos (darwin), sqlite3 is embedded using "the amalgamation". There
is therefore no need to link to the system sqlite3, and doing so can
cause unexpected behaviour/failures.
Also be explicit about darwin vs. non-darwin port_specs sources.
rebar3 seems to get confused when compiling under eunit command sometimes
This is my second/better attempt at fixing the OS X build. This build
explicitly includes the sqlite3 amalgamation files. sqlite shipped on
OS X is old. It doesn't support extensions. This change allows users to
build erlang-sqlite3 on OS X and pass tests.
This causes the port driver to use the `brew` version of SQLite on OS X
(darwin). This version is more up-to-date an includes functionality like
loading extensions.
