mingw32-git: Use the same prefixes at compile time as at install time

Git tracks changes to path prefixes (see TRACK_PREFIX in its Makefile)
and re-compiles several files at install time if a tracked prefix has
changed since compile time. Avoid this by setting all tracked prefixes at
compile time to their exact install time values. The only tracked prefix
that we did not set is "bindir", so set it to the value as specified in
src_install.mgwpart inside the mgwinstall() function.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
mingw32-git: Set the correct sysconfdir (for "git config --system")

sysconfdir is not one of the prefixes tracked by Git's Makefile so we can
freely change it at compile time and not care about install time.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
mingw32-git: Install a default system-wide gitconfig

This has http.sslCAInfo set as required by cURL if built with OpenSSL
instead of WinSSL support.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
mingw32-git: Do not use make_etc_defaults yet as postinstall is unimplemented

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>