mingw: allow building with an MSYS2 runtime v3.x

mingw: allow building with an MSYS2 runtime v3.x

Recently the Git for Windows project started the upgrade process to
a MSYS2 runtime version based on Cygwin v3.x.

This has the very notable consequence that `$(uname -r)` no longer
reports a version starting with "2", but a version with "3".

That breaks our build, as df5218b4c30b (config.mak.uname: support MSys2,
2016-01-13) simply did not expect the version reported by `uname -r` to
depend on the underlying Cygwin version: it expected the reported
version to match the "2" in "MSYS2".

So let's invert that test case to test for *anything else* than a
version starting with "1" (for MSys). That should safeguard us for the
future, even if Cygwin ends up releasing versionsl like 314.272.65536.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>