Allow IE to build with monolithic USD

SConstruct

@@ -297,6 +297,13 @@ options.Add(
 	"usd_"
 )
 
+options.Add(
+	"USD_LIB_SUFFIXES",
+	"The suffixes to append to the names of the USD libraries. You can modify this "
+	"to \"usd_ms\" to use a monolithic build of USD. ",
+	"sdf arch tf vt"
+)
+
 # general variables
 
 options.Add(
@@ -1330,7 +1337,7 @@ libraries = {
 
 	"GafferUSD" : {
 		"envAppends" : {
-			"LIBS" : [ "Gaffer", "GafferDispatch", "GafferScene", "IECoreScene$CORTEX_LIB_SUFFIX" ] + [ "${USD_LIB_PREFIX}" + x for x in [ "sdf", "arch", "tf", "vt" ] ],
+			"LIBS" : [ "Gaffer", "GafferDispatch", "GafferScene", "IECoreScene$CORTEX_LIB_SUFFIX" ] + [ "${USD_LIB_PREFIX}" + x for x in env["USD_LIB_SUFFIXES"].split( " " ) ],
 			# USD includes "at least one deprecated or antiquated header", so we
 			# have to drop our usual strict warning levels.
 			"CXXFLAGS" : [ "-Wno-deprecated" if env["PLATFORM"] != "win32" else "/wd4996" ],

config/ie/options

@@ -461,6 +461,9 @@ if usdIncludePath:
 if usdLibPath:
 	LOCATE_DEPENDENCY_LIBPATH.insert(0, usdLibPath)
 
+# ImageEngine is now using a monolithic build of USD
+USD_LIB_SUFFIXES = "usd_ms"
+
 ##########################################################################
 # get compiler and other build tools set up
 ##########################################################################