build: Enable pxr_boost::python for Python bindings

Python bindings are now built with the internal pxr_boost::python
library instead of boost::python. With this change, OpenUSD is
now boost-free.

As a temporary backwards compatibility and testing mechanism,
clients may revert to the previous use of boost::python by specifying
PXR_USE_BOOST_PYTHON=TRUE when running cmake. This option is
deprecated and will be removed in a future release.

For now, boost must still be supplied to the OpenUSD build when
OpenVDB support is enabled. This is only necessary because OpenVDB
includes boost headers in its own public headers and our build
system currently does not pick up this information up from OpenVDB
itself. OpenUSD itself does not directly use any boost functionality.

(Internal change: 2343232)

cmake/defaults/Options.cmake

@@ -26,7 +26,7 @@ option(PXR_BUILD_PYTHON_DOCUMENTATION "Generate Python documentation" OFF)
 option(PXR_BUILD_HTML_DOCUMENTATION "Generate HTML documentation if PXR_BUILD_DOCUMENTATION is ON" ON)
 option(PXR_ENABLE_PYTHON_SUPPORT "Enable Python based components for USD" ON)
 option(PXR_USE_DEBUG_PYTHON "Build with debug python" OFF)
-option(PXR_USE_BOOST_PYTHON "Use boost::python for Python bindings" ON)
+option(PXR_USE_BOOST_PYTHON "Use boost::python for Python bindings (deprecated)" OFF)
 option(PXR_ENABLE_HDF5_SUPPORT "Enable HDF5 backend in the Alembic plugin for USD" OFF)
 option(PXR_ENABLE_OSL_SUPPORT "Enable OSL (OpenShadingLanguage) based components" OFF)
 option(PXR_ENABLE_PTEX_SUPPORT "Enable Ptex support" OFF)