Since I didn't want to have two separate sources for windows and mac, I searched a bit and found (hopefully) a way to compile again with the old LLVM-GCC4.2 on Xcode 5. It was a little bit tricky, I summarize the steps below for those who may be interested, until Max'SDK is updated:
1/ install LLVM-GCC4.2: I did that using macports, since the Fink and Homebrew do not seem to allow to build it in universal i386/x86_64 (and the homebrew version is somehow suspicious). I used the following command:
$ port install apple-gcc42 +universal +gpl3
This should install the compiler at /opt/local/bin/llvm-gcc-4.2
2/ allow Xcode 5 to compile with it. For this you have to install a plugin. I did the hard work, the plugin is attached to this post, you have to decompress it and put it into "/Applications/Xcode.app/Contents/Plugins/Xcode3Core.ideplugin/Contents/SharedSupport/Developer/Library/Xcode/Plug-ins/". Not sure if it's 100% clean (I took an example on the web and modified it according to my needs), but it seems to work.
3/ In the build settings of the xcode project of the external (here "uires.xcodeproj"):
a/ "Compiler for C/C++/Objective-C" should be set to the new entry "LLVM-GCC 4.2"
b/ "Path to Linker Dependency Info File" should be unset (otherwise you get an error about ld not understanding "-dependency_info").