bsd.compiler.mk: Detect distribution-provided GCC when executed as cc

[jrtc27]

Summary

Clang always prints "clang $VERSION" regardless of the name used to
execute it, whereas GCC prints "$progname $VERSION", meaning if CC is
set to cc and cc is GCC it will print "cc $VERSION". We are able to
detect some of those cases since it then prints "($PKGVERSION)", where
the default is "GCC", but many distributions override that to print
their name and the package version number (e.g. "Debian 10.2.1-6"), so
nothing tells us it's GCC other than the fact that it's not Clang (and
that there's an FSF copyright disclaimer).

However, GCC's -v option will always print "gcc version $VERSION", so
fall back on using that to detect GCC. Whilst Clang also supports this
option, we should never get here, so Clang handling is not added.

[emaste]

This is fine with me.

Perhaps we can simplify it to always use ${${cc}:N${CCACHE_BIN}} -v (instead of --version). We'd have to make sure all existing cases still work though.

[jrtc27]

In D28315#634274, @emaste wrote:

This is fine with me.

Perhaps we can simplify it to always use ${${cc}:N${CCACHE_BIN}} -v (instead of --version). We'd have to make sure all existing cases still work though.

Yeah I think that should work just fine, it's what we use in our cheribuild tool for similar detection, this was just the easiest non-controversial targeted fix (in that it only changes code that currently emits an error, so surely there can't be any regressions!).

[emaste]

it only changes code that currently emits an error

Yeah, it's for that reason that I'm fine with it as-is if you want to leave it.