Build: upgrade to OpenEXR 2.4.0, OpenVDB 7.0.0 and Boost 1.70.0

This aligns with the VFX reference platform 2020 along with the decision
to stick to Python 3.7, see T68774.

Blosc was downgraded to 1.5 as recommended by the OpenVDB documentation.

IlmBase and OpenEXR are now built together with CMake rather separately
using autoconf.

Differential Revision: https://developer.blender.org/D6593

intern/openvdb/CMakeLists.txt

@@ -43,6 +43,7 @@ if(WITH_OPENVDB)
   if(WIN32)
     add_definitions(
       -DOPENVDB_STATICLIB
+      -D_USE_MATH_DEFINES
     )
   endif()