GUADEC: porting GNOME to Android

It is possible for a configure.ac to require this, but that feature is relatively rarely-used (most Autoconf checks only require preprocessing, compiling or sometimes linking a program for the embedded architecture, not actually running it).

When a check can't be done without running a program for the embedded architecture (e.g. dbus checks for support for Linux/Solaris-style abstract Unix sockets), in a properly written configure.ac it's possible to tell configure what the result will be, which stops it from actually trying the check. For instance, when building for Android you know that the kernel does support abstract Unix sockets, and there's a configure option to say so.