Improved support for building GafferOSL.
Improved support for building GafferOSL.

Added a build variable for specifying a custom suffix for the osl libraries, and ensured that oslc is run in an environment where ENV_VARS_TO_IMPORT is in effect, to support custom oslc wrappers.

cedriclaunay/gaffer,GafferHQ/gaffer,davidsminor/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,hradec/gaffer,hradec/gaffer,chippey/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,goddardl/gaffer,cedriclaunay/gaffer,boberfly/gaffer,davidsminor/gaffer,GafferHQ/gaffer,hradec/gaffer,ImageEngine/gaffer,boberfly/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,davidsminor/gaffer,ImageEngine/gaffer,chippey/gaffer,goddardl/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,cedriclaunay/gaffer,ivanimanishi/gaffer,hradec/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,boberfly/gaffer,chippey/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,boberfly/gaffer,chippey/gaffer,cedriclaunay/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,hradec/gaffer,andrewkaufman/gaffer,goddardl/gaffer,goddardl/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,davidsminor/gaffer