autogen.sh: add option to skip autotools invocations

This is to comply with Gentoo packaging policy: it is preferred that
Portage-provided procedures handle autotools invcations.

autogen.sh

@@ -31,6 +31,10 @@
 
   mkdir -p config
 
+  if [ "$NO_AUTOTOOLS_RUN" ]; then
+    exit 0
+  fi
+
   intltoolize --force --automake \
   && aclocal -I ./m4 \
   && $LIBTOOLIZE --copy --force --automake \