[MacPorts] #54766: Does port:clang*'s libstdcxx have to be default?

[Jeremy Huddleston Sequoia]

> Comment (by RJVB) in <https://trac.macports.org/ticket/54766#comment:16>:
> 
>> Also, if you do bump llvm-3.9 and llvm-4.0 for this, please roll in the llvm-bin changes from be6401d19706f7317979dfbd680592cee5f7cf9d, so we can do both changes with a single revbump.
> 
> I don't get that change: I'm more used to removing proxying through xcrun to make it possible to use other compilers than AppleClang. Maybe this targets newer OS versions (than my 10.9)?

As you may have noticed, over the past year, I've added support to base to better handle cases where the DevSDK (the part of the Command Line Tools package that installs /usr/include) is not installed.  Thus, executing clang-mp-5.0 (without this change) fails unless you either specify -isysroot on the command line or use xcrun.  If you do not specify an SDK to xcrun, it will determine an implied SDK (the DevSDK if present, or the "real" macOS SDK if not) and set the SDKROOT environment variable to that path.  If -isysroot is not passed to clang and the SDKROOT environment variable is set, clang uses that value.

This allows our clang ports to "just work" for users that install them without installing the DevSDK.

Removing proxying through xcrun has nothing to do with making it possible to use other compilers.