Implement pkg-config detection in autogen.sh
This patch checks the presence of the pkg-config Autoconf macro before running the configure script. Fixes #21 (closed).
Refer-to: autogen.sh should check pkg-config's presence (#21 (closed)) · Issues · GNOME / libxml2 · GitLab #21 (closed) Signed-off-by: 林博仁(Buo-ren Lin) Buo.Ren.Lin@gmail.com