Update compiler selection for 10.6-10.8 with libc++

[jmroot (Joshua Root)]

For this configuration on these systems, we need to at least default configure.cxx to a compiler that can use libc++. We probably also need some bootstrap (libstdc++ using) versions of toolchain ports so that the manual intervention described in LibcxxOnOlderSystems is not required.

[ryandesign (Ryan Carsten Schmidt)]

Possibly related to #55444 and/or #56042.

[kencu (Ken)]

how about something like this in portconfigure.tcl:

    # Determine which versions of clang we prefer
    if {${configure.cxx_stdlib} eq "libc++"} {
        # clang-3.5+ require libc++
        lappend compilers macports-clang-5.0 macports-clang-4.0

        if {${os.major} < 17} {
            # The High Sierra SDK requires a toolchain that can apply nullability to uuid_t
            lappend compilers macports-clang-3.9
        }

        if {${os.major} < 16} {
            # The Sierra SDK requires a toolchain that supports class properties
            lappend compilers macports-clang-3.7
        }

+        if {${os.major} < 13} {
+            # for older systems configured to libc++, return a current clang version
+            return macports-clang-5.0 macports-clang-4.0 macports-clang-3.9
+        }


    }

[kencu (Ken)]

So - you'll notice that this forces all these systems 10.6 through 10.8 to basically build everything with clang-5.0. I just can't help but think this makes a lot of sense, removes our need to set up complicated compiler floors with certain c++ or C standard support available, and makes all these systems build with basically the same compiler that newer systems use (and are tested against, etc).

I realize that means that clang-5.0 will need to be called in for every build on these systems. But I submit that is a better option than the alternative of seeing builds fail with older builds of clang, and forcing them all up to something newer.

All these systems will end up with clang-5.0 anyway, for some port or another.

And it allows the default -stdlib to be set to libc++ in clang-5.0 for all systems, and have this carry through to all these older systems.

[mojca (Mojca Miklavec)]

I assume that the list should still contain default compilers at the end, for bootstrapping reasons.

[jmroot (Joshua Root)]

Replying to mojca:

I assume that the list should still contain default compilers at the end, for bootstrapping reasons.

Not needed if the bootstrap compilers set their own whitelist.

[jmroot (Joshua Root)]

[5e17a692f991a55a7f450e40ce47ab6fe0f2da57/macports-base], [640b7e0aa8710d26cd0af15bde29bb6290ad6b20/macports-ports]