Don't use AC_CANONICAL_TARGET

Don't use AC_CANONICAL_TARGET

This autoconf macro should only be used for building compilers
(or compiler tools) for a specific target. The current effect of
it in GTK3 is that it causes various executables like gtk3-demo
to be prefixed with $target- when the --target configure flag
is set or when cross-compiling. When cross-compiling GTK3 on
Linux for the Win32 target this causes the gtk3-demo binary
to be named i686-w64-mingw32-gtk3-demo.exe instead of just
gtk3-demo.exe (like it was before commit 53083ea7b423482b)

https://bugzilla.gnome.org/show_bug.cgi?id=692638