Arch Linux

With the combination of Boost 1.66.0-2 and CMake
3.11.1-1 (i.e., the current repository versions), Boost Python modules
are linked to a wrong version of Boost Python when also
linked against Boost Python Numpy.

Maybe the problem is caused by CMake 3.11.1 that supports
Boost 1.67.0 (see https://github.com/Kitware/CMake/commit/1673923c303c6a4184904c4c5849911feddb87e7#diff-555801259d7df67368f7deab1f9deacd)
that changed library naming (see https://www.boost.org/doc/libs/1_67_0/libs/python/doc/html/rn.html#rn.version_1_67).
However, with Boost 1.66.0, not only `libboost_python3`, but also `libboost_python` is linked,
unfortunately, the latter corresponds to Python 2 and causes ImportError when imported:
```
E ImportError: /usr/lib/libboost_python.so.1.66.0: undefined symbol: PyString_InternFromString
```

_Maybe_ just updating boost to the newest 1.67.0 would help. If not, this should be reported
in CMake upstream.

###################
Steps to reproduce:
###################

1. Create file bp.cpp:
```
#include <boost/python.hpp>
BOOST_PYTHON_MODULE(bp)
{
}
```

2. Create file CMakeLists.txt:
```
cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
# Note that everything is fine when numpy3 is removed.
find_package(Boost REQUIRED COMPONENTS python3 numpy3)
find_package(PythonLibs 3 REQUIRED)
add_library(
bp SHARED
bp.cpp
)
set_target_properties(
bp PROPERTIES
PREFIX ""
)
target_include_directories(
bp
PRIVATE ${Boost_INCLUDE_DIRS}
PRIVATE ${PYTHON_INCLUDE_DIRS}
)
target_link_libraries(
bp
PRIVATE ${Boost_LIBRARIES}
PRIVATE ${PYTHON_LIBRARIES}
)
```

3. Run:
```
cmake . && make
python -c "import bp"
```

The import error is thrown. `ldd bp.so` shows that also `libboost_python` is linked.