Enable building with libgphoto2 version 2.4.

The installed version of gphoto2 is detected in setup.py and appropriate
pre-processor defines allow conditional code in the interface definition
files.
Some of the examples work with version 2.4, but not all.

setup.py

@@ -19,6 +19,10 @@
 
 from distutils.core import setup, Extension
 import os
+import subprocess
+
+gphoto2_version = subprocess.check_output(['gphoto2-config', '--version'])
+gphoto2_version = tuple(gphoto2_version.split()[1].split('.'))
 
 mod_names = map(lambda x: x[0],
                 filter(lambda x: x[1] == '.i',
@@ -27,11 +31,16 @@
 
 ext_modules = []
 init_module = ''
+swig_opts = ['-I/usr/include', '-builtin', '-O', '-Wall']
+if gphoto2_version[0:2] == ('2', '4'):
+    swig_opts. append('-DGPHOTO2_24')
+elif gphoto2_version[0:2] == ('2', '5'):
+    swig_opts. append('-DGPHOTO2_25')
 for mod_name in mod_names:
     ext_modules.append(Extension(
         '_%s' % mod_name,
         sources = ['source/lib/%s.i' % mod_name],
-        swig_opts = ['-I/usr/include', '-builtin', '-O', '-Wall'],
+        swig_opts = swig_opts,
         libraries = ['gphoto2', 'gphoto2_port'],
         extra_compile_args = ['-O3', '-Wno-unused-variable'],
         ))

source/lib/gphoto2_port_info_list.i

@@ -41,7 +41,9 @@
   PyList_Append($result, SWIG_NewPointerObj(*$1, SWIGTYPE_p__GPPortInfoList, 0));
 }
 
-// gp_port_info_new() returns a pointer in an output parameter
+// In libgphoto2 version 2.4 GPPortInfo is a structure, in version 2.5 it's a
+// pointer to a structure.
+#ifdef GPHOTO2_25
 %typemap(in, numinputs=0) GPPortInfo * (GPPortInfo temp) {
   $1 = &temp;
 }
@@ -53,6 +55,7 @@
   }
   PyList_Append($result, SWIG_NewPointerObj(*$1, SWIGTYPE_p__GPPortInfo, 0));
 }
+#endif
 
 // several getter functions return string pointers in output params
 %typemap(in, numinputs=0) char ** (char *temp) {