cc(_r) and xlc(_r) refer to the same binary of the same compiler, but they change the default configuration of that compiler.

The documentation indicates:

 DESCRIPTION
         The invocation commands compile C and C++ source files.
         The commands and their recommended usages are described
         below.

         Invocations                   Usage (supported standards)
         _________________________________________________________

         xlc, xlc_r, xlc_r4,           Compile C source files.
         xlc_r7, xlc128, xlc128_r,     (ANSI C89, ISO C99,
         xlc128_r4, xlc128_r7          IBM language extensions)

         xlc++, xlc++_r, xlc++_r4,     Compile C++ source files.
         xlc++_r7, xlc++128,
         xlc++128_r, xlc++128_r4,
         xlc++128_r7, xlC, xlC_r,
         xlC_r4, xlC_r7, xlC128,
         xlC128_r, xlC128_r4,
         xlC128_r7

         cc, cc_r, cc_r4, cc_r7,       Compile legacy code that
         cc128, cc128_r, cc128_r4,     does not conform to Standard
         cc128_r7                      C. (pre-ANSI C)

         c89, c89_r, c89_r4,           Compile C source files with
         c89_r7, c89_128, c89_128_r,   strict conformance to the
         c89_128_r4, c89_128_r7        C89 standard. (ANSI C89)

         c99, c99_r, c99_r4,           Compile C source files with
         c99_r7, c99_128, c99_128_r,   strict conformance to the
         c99_128_r4, c99_128_r7        C99 standard. (ISO 99)

         xlc++core, xlc++core_r,       Compile C++ source files
         xlc++core_r7, xlc++core128,   but the compiler will
         xlc++core128_r,               link only to the core of
         xlc++core128_r7, xlCcore,     the runtime library.
         xlCcore_r, xlCcore_r7,
         xlC128core, xlC128core_r,
         xlC128core_r7

         gxlc                          Accepts GNU C options, maps
                                       them to their equivalent XL
                                       C option, then invokes xlc.

         gxlc++, gxlC                  Accepts GNU C/C++ options, maps
                                       them to their equivalent XL
                                       C++ option,then invokes xlc++.

         The main difference between these commands is that they
         use different default options (which are set in the
         configuration file /etc/vac.cfg.XX, where XX represents
         the AIX version).
         See the Compiler Reference for more information on these
         invocation commands.

         All invocations with a suffix of _r allow for thread-safe
         compilation. Use these commands to create threaded
         applications or to link programs that use multi-threading.

So xlc_r seems like the right one as the default to compile Python I would say.

I will check your last commit soon (I have buildbot nearly running on AIX, that is how I observed this problem actually).