build: Switch to olddir after autoreconf

Otherwise this won't work:

  $ mkdir _build && cd _build
  $ ../autogen.sh

As `autoreconf` won't find configure.ac.

autogen.sh

@@ -18,7 +18,6 @@ fi
 # regenerated from their corresponding *.in files by ./configure anyway.
 touch INSTALL
 
-cd "$olddir"
 if ! test -f libglnx/README.md; then
     git submodule update --init
 fi
@@ -27,4 +26,5 @@ sed -e 's,$(libglnx_srcpath),'${srcdir}/libglnx,g < libglnx/Makefile-libglnx.am
 
 autoreconf --force --install --verbose || exit $?
 
+cd "$olddir"
 test -n "$NOCONFIGURE" || "$srcdir/configure" "$@"