Update paths for iOS 5.1

These values can be overridden with some poorly documented and
overloaded options: --libc and --sdk-path

../libvpx/configure --target=armv7-darwin-gcc --sdk-path=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer --libc=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/

So for someone who still wants to build with the iOS 5 SDK, the last
part of the path should be iPhoneOS5.0.sdk

Change-Id: Ibe93d96ae828c619700dc3222983aa4c30456b88

build/make/configure.sh

@@ -813,7 +813,8 @@ process_common_toolchain() {
 
         darwin*)
             if [ -z "${sdk_path}" ]; then
-                SDK_PATH=/Developer/Platforms/iPhoneOS.platform/Developer
+                SDK_PATH=`xcode-select -print-path 2> /dev/null`
+                SDK_PATH=${SDK_PATH}/Platforms/iPhoneOS.platform/Developer
             else
                 SDK_PATH=${sdk_path}
             fi
@@ -835,7 +836,7 @@ process_common_toolchain() {
             add_ldflags -arch_only ${tgt_isa}
 
             if [ -z "${alt_libc}" ]; then
-                alt_libc=${SDK_PATH}/SDKs/iPhoneOS5.0.sdk
+                alt_libc=${SDK_PATH}/SDKs/iPhoneOS5.1.sdk
             fi
 
             add_cflags  "-isysroot ${alt_libc}"