Use sdk.prf to set macosx-version-min instead of static conf files

Allows us to dynamically generate the command line option for iOS later,
and allows the user to override QMAKE_MACOSX_DEPLOYMENT_TARGET with the
expected effect on the command line options.

We unset PERL5LIB to ensure we get the system Perl libraries, since the
Mac OS 10.6 CI machine seems to have a broken XML::Parser::Expat from
macports/CPAN.

Change-Id: I04430c7b1daf9452d72f9a04a6b7f8d0d6926884
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>