git build system will trigger a rebuild if it detects
if flags passed to make changed.
In commit 82a5337c07, ("git: correct utf-16 and utf-32 conversion on
musl", 2019-10-30) , we tried to correct the git-iconv interaction on
musl, but we forget to pass that flag into `make install'.
Hence, on do_install, git build system rebuild git without
ICONV_OMITS_BOM=Yes flag, thus produce faulty binary.
Save that flag into config.mak[1] in order to keep it consistent across
build stage.
While we're at it, also move other make_*_args into config.mak
[1]: https://public-inbox.org/git/20191031181116.GC2133@sigill.intra.peff.net/
7556428da1