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the updated patch addresses:
 - fix typo mentioned in msg156044
 - don't use the host python during build and installation
 - for include files and libraries, use the search directories
 provided by gcc, and only fall back to hard coded
 directories for native builds.
 - fix the dpkg-architecture call for cross builds
 - pass the --host and --build arguments to the libffi build
 - fix the pydebug check, taken from issue #3871.
The patch introduces two new variables, which are set in the environment for cross builds:
 - _PROJECT_BASE is used by Lib/sysconfig.py to point to the
 current source, while using the python3 interpreter of the
 build machine, so that the Makefile and the pyconfig.h are
 read from the current sources.
 - _HOST_PLATFORM is used to override the get_platform calls
 in Lib/sysconfig.py, Lib/distutils/utils.py and setup.py.
 Yes, distutils is frozen, however this change shouldn't
 break anything else. It only relies on one environment
 variable _HOST_PLATFORM, so if this is an issue, then the
 name can be obfuscated even more.
tested with a cross build for arm-linux-gnueabihf on x86_64-linux-gnu, and a native build.