gettext: use a GIT_LOCALE_PATH relative to $(prefix)

On Windows, we simply pass a POSIX path to bindtextdomain(), relying on
the current libintl-8.dll implementation to handle that gracefully by
resolving the path relative to the "root" directory inferred from the
location of the .dll file itself.

However, not only does this rely on the custom patches of the gettext
library as shipped with MSYS2 (gettext's own source code is not prepared
to handle POSIX paths on Windows), it also means that Git itself cannot
use the `podir` variable at all because it does not handle absolute
POSIX paths in system_path() correctly, leaving them as-is.

This patch fixes that behavior by always using a GIT_LOCALE_PATH
relative to the (runtime) prefix.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho