.jhbuildrc-custom needs update to support clean OSX 10.8.x environment

[Max Arai (developer)]

The section of .jhbuildrc-custom looks for a 10.5 SDK.  A new OSX 10.8.x environment installs Xcode 4.6.3, and the command line tools only have SDKs for OSX 10.7 and OSX 10.8

To be consistent with the .jhbuildrc-custom supplied with the GTK bootstrap, the section which currently reads
(https://github.com/jralls/gnucash-on-osx/blob/master/.jhbuildrc-custom)

#Setup the build. PPC users will need to replace "i386" with
#"ppc". Don't try to do a universal build, it doesn't work. x86_64 is
#untested, as is building against the 10.6 or 10.7 SDKs.
setup_sdk("10.5", "10.5",["i386"])

could be replaced with corresponding code from the GTK supplied .jhbuildrc-custom:

#  Set up a particular target and SDK: For default operation, set the                                                                                        
# architecture and SDK for the native machine:                                                                                                               
_target = None;
if _osx_version >= 7.0:
    _target = "10.7"
elif _osx_version >= 6.0:
    _target = "10.6"
elif _osx_version >= 5.0:
    _target = "10.5"
elif _osx_version >= 4.0:
   _target = "10.4"

setup_sdk(target=_target, sdk_version="native", architectures=[_default_arch])

If this is not appropriate, please let me know.  If it looks ok, how does one go about updating the build process?

All the best,
Max