[AD] fix for universal mac build on PPC

["Matthew Leverton" <meffer@xxxxxxxxxx>]

The macosx-universal target does build on PPC. The reason is that it
uses the wrong SDK for the Intel build. The attached patch forces it
to use the (required) 10.4u SDK.

The following switch is added to the various UB flags:

-isysroot /Developer/SDKs/MacOSX10.4u.sdk

Via testing, it appears that it must be present on the DYLIB/C/L FLAG
options. I think the hardcoded path is safe. If not, it could always
be moved into a variable for an easier change.

I believe this means that the universal build will require a minimum
of OS X 10.4 for both PPC and Intel. However, I'm not sure if there is
any clean way around that without coming up with a brand new build
system. From my (limited) understanding, in order to target different
versions of OS X for the PPC and Intel builds within a makefile, you'd
have to build the project twice and then use "lipo" to combine both
binaries into one.

(Thomas Harte is working on a more friendly binary version for the
XCode IDE that may have better backward compatibility for PPC. I'm
sure he'll report his findings after he's done.)

Regardless of the legacy OS X compatibility concerns, the patch in
some form should be safe to commit, as the universal build currently
is completely broken on PPC.

--
Matthew Leverton

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