should use absolute path for /usr/bin/which

This fixes the issue where xcodebuild spews a bunch of errors at start
that look like ...

```
error: Error Domain=NSPOSIXErrorDomain Code=2 "Non-zero exit code 1
returned from shell command:
/Users/fpotter/.../Vendor/clang-as-ios-dylib/links/cc-iphonesimulator-latest-targeting-earliest
-v -E -dM -arch x86_64 -isysroot
/Applications/xcode_5.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk
-x objective-c -c /dev/null 2>&1" UserInfo=0x7f93de1401a0
{NSLocalizedDescription=Non-zero exit code 1 returned from shell
command:
/Users/fpotter/.../Vendor/clang-as-ios-dylib/links/cc-iphonesimulator-latest-targeting-earliest
-v -E -dM -arch x86_64 -isysroot
/Applications/xcode_5.1.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk
-x objective-c -c /dev/null 2>&1, NSLocalizedFailureReason=No such file
or directory}
```