Make osxmakeitoctave3.m compatibe with Octave-Forge's Octave-3.8.0 release.

Requires definition of different -rpath’es. We do so and drop support
for Octave 3.6.x.
Requires some replacement of … line continuation by \ continuation.
Move the “-W, -mmacosx-version-min=’10.6’” statements out and into the
mkoctfile of the octave installation used for building. For some
reason, 3.8.0’s mkoctfile can’t parse our -W — compiler option
statements anymore.
Hack octave-3.8.0’s mkoctfile to use gcc and g++ as compilers and
linkers, instead of the bundled gcc-mp-4.7 or such, which has trouble
with some Apple’isms in OSX headers, e.g., for some IOUSB includes as
needed by PsychHID.

-> All mex files for 64-Bit Octave 3.8.0 on OSX rebuilt and basic
tested.
-> End of support for Octave 3.6 and earlier on OSX.