fish wants to build with -mmacosx-version-min=10.9. This is important
because it ensures that we do not use functions or linker features which
which are not available on 10.9. However this collides with the fact
that fish also prefers to use a pcre2 package installed on the system,
which is typically built for that system.
Mac ld will (rightly) complain when it sees a 10.9-targeted binary
linking a 10.15-targeted dylib. This is an annoying warning that gets
emitted on every build.
We could fix this either having Mac builds prefer the vendored PCRE2
by default, or by having debug builds target the system version. But
we want to continue to default to system PCRE2 and we don't want to risk
losing compatibility with older Mac versions. So we will just suppress
all linker warnings in Mac debug builds.
Building on OS X versions prior to 10.13.6 fails at the very end when
running `codesign`.
The `-options runtime`-argument isn't available on these earlier
versions of the OS.
Simply running codesign without that argument (on OS X <10.13.6) seems
to produce a runnable binary with no security warnings.
