I'm trying to compile GDC on Mac OSX, and I hit the "Not
Implemented" assertion in thread.d of phobos:

                  pop RBX;
                  pop RBP;

                  // 'return' to complete switch
                  pop RCX;
                  jmp RCX;
              }
          }
          else static if( __traits( compiles, ucontext_t ) )
          {
              Fiber   cfib = Fiber.getThis();
              void*   ucur = cfib.m_ucur;

              *oldp = &ucur;
              swapcontext( **(cast(ucontext_t***) oldp),
                            *(cast(ucontext_t**)  newp) );
          }
          else
              static assert(0, "Not implemented");
      }
}
(line 3365);

I am using:
   - latest GDC code from Github, branch gdc-4.9
(e748df611430b497e6147309fbec4aad11e4ae26).
   - Mac OSX 10.9.4.
   - GCC 4.9.1 installed via homebrew, I am setting export CC=...
and export CXX=....
   - GCC 4.9.1 sources downloaded from a mirror as per the
instructions in http://wiki.dlang.org/GDC/Installation/Generic.
   - Configure command: ../../gcc-4.9.1/configure
--enable-languages=d --disable-bootstrap --prefix=/usr/local/gdc
--with-bugurl="http://bugzilla.gdcproject.org"
--enable-checking=yes
   - make CFLAGS='-w' CXXFLAGS='-w'

Am I doing something wrong, or is GDC currently uncompilable on
Mac?

Sorry, GDC's directory name 'libphobos' is misleading, the error is in druntime's thread.d. phobos doesn't have a thread.d.