python: Fix incorrect handling of PXR_PY_UNDEFINED_DYNAMIC_LOOKUP

pxr_boost::python was bypassing this setting by specifying
Python3::Python as a dependency instead of PYTHON_LIBRARIES,
which the build system sets up while taking the above setting
into account.

PYTHON_INCLUDE_DIRS must also be specified as an include
directory so that Python headers are still picked up
during the build, even if we're not linking against the
Python shared library.

This fixes crashes when using the usd-core Python module
from PyPI on MacOS.

(Internal change: 2343414)

pxr/external/boost/python/CMakeLists.txt

@@ -10,7 +10,11 @@ set(cppfiles "")
 # Otherwise, this library is just a pass-through for boost::python.
 if (PXR_USE_INTERNAL_BOOST_PYTHON)
     set(libs
-        Python3::Python
+        ${PYTHON_LIBRARIES}
+    )
+
+    set(include_dirs
+        ${PYTHON_INCLUDE_DIRS}
     )
 
     set(cppfiles