GDC on OSX - compilation error

[amirabiri via D.gnu]

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?