Fixed OSL code for GCC 4.1.2.

Also corrected some #include statements to include the OSL prefix.
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.
Merge pull request #802 from johnhaddon/oslBuildImprovements

OSL build improvements