Patch LibQxt to make it compatible to libc++

The original configure script hard-codes the build spec to macx-g++, and generates a build linked against libstdc++. This makes it incompatible to Homebrew Qt on Mavericks. A patch is added to remove the line so that the configure script respects the default spec.

The main reason LibQxt sets the spec explicitly is that the default Qt spec for OS X (macx-xcode) generates an Xcode project. This is not a problem here because Homebrew Qt always uses a Makefile-based spec anyway.