Windows : Disable some warnings from external dependencies.

- MSVC seems to be inconsistent with considering headers included via double quotes as external. Rather than breaking with convention and including external dependencies in angle brackets, we disable these warnings until MSVC handles external warnings correctly.
- See https://developercommunity.visualstudio.com/t/analyze:external--and-external:I-flags/1688240#T-N1689542 for additional information. They are using `/analyze` but it seems to apply to compiling as well.

Author: ericmehl

SConstruct

@@ -1179,7 +1179,6 @@ else:
 			"/D_WINDLL",
 			"/D_MBCS",
 			"/W4",
-			"/experimental:external",
 			"/external:W0",
 			"/Zc:inline", # Remove unreferenced function or data if it is COMDAT or has internal linkage only
 			"/GR", # enable RTTI
@@ -1204,6 +1203,13 @@ else:
 				"/wd4244",  # suppress warning about possible loss of data in type conversion
 				"/wd4305",  # suppress warning about conversion from double to float
 				"/wd4506",  # suppress warning about no definition for inline function. Needed for USD::Glf
+				# NOTE : the following warnings are generated by external dependencies, even when using /external
+				# They may be able to be re-enabled after updating to MSVC 2022.
+				"/wd4127",  # suppress warning "conditional expression is constant", Needed for USD
+				"/wd4456",  # suppress warning "declaration of 'x' hides previous local declaration"
+				"/wd4459",  # suppress warning "declaration of 'x' hides global declaration"
+				"/wd4201",  # suppress warning "nonstandard extension used : nameless struct/union"
+				"/wd4245",  # suppress warning "'initializing': conversion from 'int' to 'size_t', signed / unsigned mismatch
 				# suppress warning about exported class deriving from non-exported class.
 				# Microsoft states (in https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-2-c4275?view=msvc-170)
 				# that "C4275 can be ignored if you are deriving from a type in the