Fix T43561: Wrong include path to standard OSL headers

Issue was in fact only visible in certain circumstances:

- OSL was compiled with Boost Wave
- or system's cpp didn't handle space between -I and path

Now made it so both wave and cpp code paths are always happy.

Original patch from Shane Ambler with own modifications to
mimic what variable holds on more verbose.

intern/cycles/render/osl.cpp

@@ -264,8 +264,8 @@ bool OSLShaderManager::osl_compile(const string& inputfile, const string& output
 	options.push_back(outputfile);
 
 	/* specify standard include path */
-	options.push_back("-I");
-	options.push_back(shader_path);
+	string include_path_arg = string("-I") + shader_path;
+	options.push_back(include_path_arg);
 
 	stdosl_path = path_get("shader/stdosl.h");