BUILD: autogen.sh: avoid calling ./configure if $NOCONFIGURE is set
BUILD: autogen.sh: avoid calling ./configure if $NOCONFIGURE is set

For a use case, see https://github.com/cgwalters/build-api

Signed-off-by: Tim Janik <6ec6ff229c4e1c5bbd5933ee92e6c5832ff023bb@gnu.org>

tim-janik/beast,GNOME/beast,GNOME/beast,tim-janik/beast,tim-janik/beast,GNOME/beast,swesterfeld/beast,tim-janik/beast,GNOME/beast,GNOME/beast,GNOME/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast,swesterfeld/beast,GNOME/beast,tim-janik/beast,swesterfeld/beast,swesterfeld/beast,tim-janik/beast,swesterfeld/beast