HYDRA-700 : Keep BOOST_PYTHON_DEBUG in MSVC builds (+explanation)

cmake/compiler_config.cmake

@@ -128,6 +128,13 @@ function(mayaHydra_compile_config TARGET)
         target_compile_definitions(${TARGET} 
             PRIVATE
                 ${MSVC_DEFINITIONS}
+                # In debug builds, Boost's wrap_python.hpp will #undef _DEBUG when BOOST_DEBUG_PYTHON is not defined,
+                # and will then include pyconfig.h :
+                # https://github.com/boostorg/python/blob/boost-1.81.0/include/boost/python/detail/wrap_python.hpp#L23-L57
+                # pyconfig.h will then autolink the Python lib; however, since _DEBUG was #undef'd, it will try to
+                # link against the release version of the Python lib, causing a linker error :
+                # https://github.com/python/cpython/blob/v3.11.4/PC/pyconfig.h#L269-L286
+                $<$<STREQUAL:${CMAKE_BUILD_TYPE},Debug>:BOOST_DEBUG_PYTHON>
         )
     endif()