[boost-python] python39_d.dll not found when BOOST_DEBUG_PYTHON is defined in a custom triplet

[klalumiere]

Is your feature request related to a problem? Please describe.
As a workaround to #5097, I decided to define BOOST_DEBUG_PYTHON in our custom triplet, which is the official way to use Python Debug libraries with Boost (this could be seen as a fix to #5097, but I'm going to let the author of the issue decide if it is). However, linking failed saying that python39_d.dll could not be found. Investigations lead me to the conclusion that the B2 option python-debugging should be set to on when BOOST_DEBUG_PYTHON is defined.

Proposed solution
Building on #15274, we can add python-debugging=on to B2_OPTIONS_DBG when BOOST_DEBUG_PYTHON is inside the VCPKG_CXX_FLAGS_DEBUG. A pull request is coming.

Describe alternatives you've considered
I see no alternative to fix the current issue. However, the initial goal was to find a workaround to #5097.

As a workaround to #5097, one alternative is to not use BOOST_DEBUG_PYTHON, i.e. to always link to the release library python39.dll. However, as frivard-coveo commented, when trying to do this in a build where the CMAKE_BUILD_TYPE is Debug, the script applocal.ps1 did not copy python39.dll inside the binaries directory because it searches for DLLs inside vcpkg_installed/x64-windows/debug. Hence, one alternative is to modify the port python3 so that the release library python39.dll is copied inside vcpkg_installed/x64-windows/debug. This is, to say the least, unconventional. Another alternative is to modify applocal.ps1 so that it can search inside both vcpkg_installed/x64-windows/debug and vcpkg_installed/x64-windows when the CMAKE_BUILD_TYPE is Debug. This alternative seems to me like a significant change, which we may want to think about before doing.