Fix libboost_python detection for boost1.71.

[sebastic]

The libboost-python-dev package in Debian changed from containing:

libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python.a
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python.so
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python27.a
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python27.so
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python3-py38.a
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python3-py38.so
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python3.a
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python3.so
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python38.a
libboost-python1.67-dev: /usr/lib/x86_64-linux-gnu/libboost_python38.so

To only containing:

libboost-python1.71-dev: /usr/lib/x86_64-linux-gnu/libboost_python38.a
libboost-python1.71-dev: /usr/lib/x86_64-linux-gnu/libboost_python38.so

[snickell]

Based on my testing, this patch also fixes the auto-detection of boost_python on OSX homebrew, and Ubuntu.

@artemp any chance you could check this patch out and consider merging? Since self-compilation is required, and this fix makes the build work again on OSX, Debian and Ubuntu, many users will be positively impacted by merging.

It would also merge, and fix build on OSX at least (and I think Ubuntu?), on the 3.0.x branch.

[snickell]

OSX homebrew:

➜  lib python3 -c "import platform ; print(platform.system())"
Darwin
➜  lib ls libboost_python*
libboost_python38-mt.a     libboost_python38-mt.dylib libboost_python38.a        libboost_python38.dylib
➜  lib pwd
/usr/local/lib
➜  lib   

[artemp]

@sebastic @snickell - merged, thanks