libs/windows: Prevent USD exports from leaking into blender binary.

Even though we build USD as static, it still feels the need to mark its
symbols with declspec(dllexport) which means the blender binary now exports
these symbols.

this patch fixes that unwanted behaviour, however USD libs still need to
rebuild before this becomes visible in the blender binary

Differential Revision: https://developer.blender.org/D6563

Reviewed By: sybren

build_files/build_environment/patches/usd.diff

@@ -121,3 +121,19 @@ diff -Naur external_usd_orig/pxr/base/lib/tf/preprocessorUtils.h external_usd/px
      #define TF_PP_IS_TUPLE(sequence) \
          BOOST_VMD_IS_TUPLE(sequence)
  #else
+diff -Naur external_usd_base/cmake/macros/Public.cmake external_usd/cmake/macros/Public.cmake
+--- external_usd_base/cmake/macros/Public.cmake	2019-10-24 14:39:53 -0600
++++ external_usd/cmake/macros/Public.cmake	2020-01-11 13:33:29 -0700
+@@ -996,6 +996,12 @@
+     foreach(lib ${PXR_OBJECT_LIBS})
+         string(TOUPPER ${lib} uppercaseName)
+         list(APPEND exports "${uppercaseName}_EXPORTS=1")
++        # When building for blender, we do NOT want to export all symbols on windows.
++        # This is a dirty hack, but USD makes it impossible to do the right thing
++        # with the default options exposed.
++        if (WIN32)
++            list(APPEND exports "PXR_STATIC=1")
++        endif()
+     endforeach()
+     foreach(lib ${PXR_OBJECT_LIBS})
+         set(objects "${objects};\$<TARGET_OBJECTS:${lib}>")