Arnold output driver : Don't link to libai.so

This was causing crashes at render shutdown in Gaffer unless we either used `RTLD_GLOBAL`
or we used `LD_PRELOAD=libai.so`. The crashes were always during deallocation, so I suspect
that we were getting mismatched allocation/deallocation between Arnold's allocator and our own.

Author: johnhaddon

SConstruct

@@ -2995,6 +2995,10 @@ arnoldPythonModuleEnv.Append(
 arnoldDriverEnv = arnoldEnv.Clone( IECORE_NAME = "ieOutputDriver" )
 arnoldDriverEnv["SHLIBPREFIX"] = ""
 arnoldDriverEnv["SHLIBSUFFIX"] = ".so" if env["PLATFORM"] != "win32" else ".dll"
+if env["PLATFORM"]=="darwin" :
+	# Symbols we need from `libai.dylib` will be resolved at runtime when Arnold
+	# loads the driver.
+	arnoldDriverEnv.Append( LINKFLAGS = "-Wl,-undefined,dynamic_lookup" )
 
 haveArnold = False
 
@@ -3041,7 +3045,6 @@ if doConfigure :
 
 		arnoldDriverEnv.Append(
 			LIBS = [
-				"ai",
 				os.path.basename( coreEnv.subst( "$INSTALL_LIB_NAME" ) ),
 				os.path.basename( imageEnv.subst( "$INSTALL_LIB_NAME" ) ),
 				os.path.basename( arnoldEnv.subst( "$INSTALL_LIB_NAME" ) ),