Boost.Python doesn't build with Python3.6 on Centos7 (scl rh-python36)

On CentOS7 with rh-python36 (scl), boost.python fails to build because the header path appears to be truncated.

Here's a relevant snippet from the output:
```
"g++" -fvisibility-inlines-hidden -fPIC -m64 -pthread -O3 -finline-functions -Wno-inline -Wall -fvisibility=hidden -DBOOST_ALL_NO_LIB=1 -DBOOST_PYTHON_SOURCE -DNDEBUG -I"." -I"/opt/rh/rh-python36/root/usr/include/python3.6" -c -o "bin.v2/libs/python/build/gcc-7.3.1/release/python-3.6/threading-multi/visibility-hidden/exec.o" "libs/python/src/exec.cpp"

...failed gcc.compile.c++ bin.v2/libs/python/build/gcc-7.3.1/release/python-3.6/threading-multi/visibility-hidden/exec.o...
gcc.compile.c++ bin.v2/libs/python/build/gcc-7.3.1/release/python-3.6/threading-multi/visibility-hidden/object/function_doc_signature.o
In file included from ./boost/python/detail/prefix.hpp:13:0,
from ./boost/python/converter/registrations.hpp:8,
from libs/python/src/object/function_doc_signature.cpp:9:
./boost/python/detail/wrap_python.hpp:50:11: fatal error: pyconfig.h: No such file or directory
# include
^~~~~~~~~~~~
compilation terminated.
```

Apparently python36 is installed in '/opt/rh/rh-python36/root/usr/include/python3.6m' and not '/opt/rh/rh-python36/root/usr/include/python3.6'

Why RH decided to append the 'm', I have no idea. I also don't know why Boost.Build is lopping that off. I have to imagine the fix is relatively simple though.