Makefile: Allow appending Git and libgit2 build options

When executing `make git`, then Gitaly provides a default set of build
options. These include all kinds of things, but most importantly we
disable a bunch of options which we don't need at runtime. In case
downstream distributions of Gitaly want to build Git with a bunch of
additional options though, they have no other way to do so than to copy
the default set of options and then append whatever they want to change
to this set. This is quite fragile, and any changes done in Gitaly won't
be reflected in downstream distributions like this.

Improve this by adding two new build variables GIT_APPEND_BUILD_OPTIONS
and LIBGIT2_APPEND_BUILD_OPTIONS, which allow appending additional
options without overriding the defaults.