macports linker with XCode 7: aka ld: library not found for-lgcc_s.10.4

One of the learnings of https://trac.macports.org/ticket/49273

If you are using gcc or clang from MacPorts, notice that whenever
Apple releases new XCode command line tools, the MacPorts will
inevitably lag until Apple opensources their changes.  This may
lead into for example link time failures, for example

   ld: library not found for -lgcc_s.10.4

is what happened with XCode 7 command line tools: Apple introduced
a new linker format called "tbd" (not making this up) which the macports
side doesn't yet know what to do with.

During this window of brokenness (if you really want to use the latest
XCode command line tools) you may need to temporarily use the the xcode
variant of the linker ld64 by

    sudo port install ld64 +ld64_xcode

and then when macports gets the support for the latest tools switch back 
with

    sudo port install ld64 +ld64_latest

Note: this should be only a temporary fix, because of XCode 7.

• macports linker with XCode 7: aka ld: library not found for-lgcc_s.10.4 by Jarkko Hietaniemi