Added LINKFLAGS option to SConstruct

Without this in SConstruct, SCons won't respect if LINKFLAGS is specified in a config file.

We use this to hardcode shared library search paths into the binaries, like this:

LINKFLAGS=[
    "-Wl,-rpath,%sboost/%s/lib"  % (libs, boost),
    "-Wl,-rpath,%scortex/%s/lib"  % (libs, cortex),
    "-Wl,-rpath,%stbb/%s/lib"  % (libs, tbb),
    "-Wl,-rpath,%soiio/%s/lib"  % (libs, oiio),
    "-Wl,-rpath,%socio/%s/lib"  % (libs, ocio),
    "-Wl,-rpath,%sqt/%s/lib"  % (libs, qt),
    "-Wl,-rpath,%spyside/%s/lib"  % (libs, pyside),
    "-Wl,-rpath,%spython/%s/lib"  % (libs, python),
]   

this way we don't need to rely on LD_LIBRARY_PATH so much.