deps_builder: Fix boost::python linking release python library

By default boost::python in debug configuration links the release
python libraries. Which leads to loading issues with the produced
modules in a debug blender (which does use the debug python libs)

Bjam has an option to switch this to the debug libraries but when
you switch that on it changes the library names for *all* boost
libraries, even ones that don't have anything to do with python
and even the release libraries.

therefore an alternative way has been chosen and we're dealing
with this by adding a define rather than asking bjam to do it.

build_files/build_environment/cmake/boost.cmake

@@ -56,6 +56,12 @@ if(WITH_BOOST_PYTHON)
     --with-python
     --user-config=${JAM_FILE}
   )
+  if(WIN32 AND BUILD_MODE STREQUAL Debug)
+    set(BOOST_PYTHON_OPTIONS
+      ${BOOST_PYTHON_OPTIONS}
+      define=BOOST_DEBUG_PYTHON
+    )
+  endif()
 endif()
 
 set(BOOST_OPTIONS