python: Remove the -builtin flag for SWIG.

* lang/python/setup.py.in: Call SWIG without the builtin flag.
--

The SWIG documentation
<http://www.swig.org/Doc2.0/Python.html#Python_nn28> leaves the
impression that -builtin is solely for increasing performance:

    New in SWIG version 2.0.4: The use of Python proxy classes has
    performance implications that may be unacceptable for a high-
    performance library. The new -builtin option instructs SWIG to
    forego the use of proxy classes, and instead create wrapped types as
    new built-in Python types. When this option is used, the following
    section ("Proxy classes") does not apply. Details on the use of the
    -builtin option are in the Built-in Types section.

While not wasting CPU cycles is good, it also prevents Python code being
written in the wrapper itself. That, however, may be useful to make it
easier to extend the wrapper.

Partially reverts: 856bcfe293

Signed-off-by: Tobias Mueller <muelli@cryptobitch.de>

lang/python/setup.py.in

@@ -154,7 +154,7 @@ class BuildExtFirstHack(build):
 
 py3 = [] if sys.version_info.major < 3 else ['-py3']
 swige = Extension("gpg._gpgme", ["gpgme.i", "helpers.c"],
-                  swig_opts = ['-threads', '-builtin',
+                  swig_opts = ['-threads',
                                '-outdir', 'gpg'] + py3 + extra_swig_opts,
                   include_dirs = include_dirs,
                   define_macros = define_macros,