autogen.sh: rework to be more buildsystem friendly

One of the goals laid out in
http://people.gnome.org/~walters/docs/build-api.txt
is to get projects to have autogen.sh scripts that
don't invoke configure after generating it.

This commit does that along with some other small
modifications designed to help be more confromant
with the Build API referenced in the above url.

https://bugs.freedesktop.org/show_bug.cgi?id=41629

autogen.sh

@@ -1,5 +1,7 @@
 #!/bin/sh
 
-(cd $(dirname $0);
- autoreconf --install --symlink &&
- ./configure $@)
+cd $(dirname $0)
+autoreconf --install --symlink
+if test -z "$NOCONFIGURE"; then
+   exec ./configure $@
+fi