rpm: Hack fix finding libpython when compiling for Python 3

Currently, building RPM with `python = python3` causes this:

checking for a Python interpreter with version >= 2.6... python3
checking for python3... /nix/store/dykqxnrwiz9drlcv2wy8lpvl3xvklx0g-python3-3.4.3/bin/python3
checking for python3 version... 3.4
checking for Python.h... yes
checking for library containing Py_Main... no
configure: error: missing python library

That comes from this snippet in configure.ac:
    AC_SEARCH_LIBS([Py_Main],[python${PYTHON_VERSION} python],[
      WITH_PYTHON_LIB="$ac_res"
    ],[AC_MSG_ERROR([missing python library])
    ])

So it's looking for (e.g) `libpython3.4.so` wheras we have `libpython3.4m.so`.
Patching the configure script to match seems to make that work (although
I don't really understand what the heck is this 'm' business about).