build fails for xcode 4.6

When building on a machine with XCode 4.6, the libgpg-error-1.10 build fails:

```
configure: WARNING: If you wanted to set the --build type, don't use --host.
    If a cross compiler is detected then cross compile mode will be used.
configure: WARNING: using cross tools not prefixed with host triplet
In file included from ./mkerrcodes.c:24:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/include/stdio.h:67:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/include/_types.h:27:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/include/sys/_types.h:33:
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/include/machine/_types.h:32:10: fatal error: 'i386/_types.h' file not found
#include "i386/_types.h"
         ^
1 error generated.
make[2]: *** [mkerrcodes] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
```

It took some doing, but I figured out that the ```SDKROOT``` environment variable set by build-all.sh screws up something in the configure script (but only with XCode 4.6).  Changing it to something else (for example, ```BUILD_SDKROOT```) fixes the problem.

This pull request does that.