Add -D_USE_MATH_DEFINES when building on Windows

This define is needed on Windows to enable the math constants
like M_PI_2 that are used in openvdb headers, otherwise builds
will fail. This is needed for OpenVDB versions after v7.0.0.
(see commit d87bb3bb in the openvdb GitHub repo).

(Internal change: 2239319)

pxr/imaging/hioOpenVDB/CMakeLists.txt

@@ -7,6 +7,12 @@ if (NOT ${PXR_BUILD_GPU_SUPPORT})
     return()
 endif()
 
+if (WIN32)
+    # OpenVDB uses constants from <cmath> that aren't available on
+    # Windows unless this is defined.
+    add_definitions(-D_USE_MATH_DEFINES)
+endif()
+
 pxr_library(hioOpenVDB
     LIBRARIES
         ar