libvpx's configure script hard-codes the SDK location of previous Xcode release in the /Developer folder. However, starting with Xcode 4.3, the SDKs moved to /Applications/Xcode.app/blabla
VLC's contrib system is clever enough to detect this, but libvpx fails miserably. However, they are providing a work-around for iOS and Android, which is expanded by this patch.

diff -ru libvpx/build/make/configure.sh libvpx/build/make/configure.sh
--- libvpx/build/make/configure.sh	2012-06-08 10:26:47.000000000 +0200
+++ libvpx-fixed/build/make/configure.sh	2012-06-08 10:26:07.000000000 +0200
@@ -628,6 +628,9 @@
     if [ -d "/Developer/SDKs/MacOSX10.7.sdk" ]; then
         osx_sdk_dir="/Developer/SDKs/MacOSX10.7.sdk"
     fi
+    if [ -d "${sdk_path}" ]; then
+        osx_sdk_dir=${sdk_path}
+    fi
 
     case ${toolchain} in
         *-darwin8-*)
