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.