ci: switch linux-musl to use Meson
Switch over the "linux-musl" job to use Meson instead of Makefiles. This is done due to multiple reasons: - It simplifies our CI infrastructure a bit as we don't have to manually specify a couple of build options anymore. - It verifies that Meson detects and sets those build options automatically. - It makes it easier for us to wire up a new CI job using zlib-ng as backend. One platform compatibility that Meson cannot easily detect automatically is the `GIT_TEST_UTF8_LOCALE` variable used in tests. Wire up a build option for it, which we set via a new "MESONFLAGS" environment variable. Note that we also drop the CC variable, which is set to "gcc". We already default to GCC when CC is unset in "ci/lib.sh", so this is not needed. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>