install pkg-config configuration files

Suggested by Jeroen Meijer.

* spot/libspot.pc.in, spot/ltsmin/libspotltsmin.pc.in: New file.
* spot/Makefile.am, spot/ltsmin/Makefile.am: Distribute them, and
install their derived version.
* spot/.gitignore: Ignore *.pc files.
* debian/libbddx-dev.install, debian/libspot-dev.install: Ship
those *.pc files.
* NEWS: Mention it.

spot/libspot.pc.in

@@ -0,0 +1,12 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+includedir=@includedir@
+libdir=@libdir@
+
+Name: Spot
+Description: A library of LTL and omega-automata algorithms for model checking
+URL: https://spot.lrde.epita.fr/
+Version: @PACKAGE_VERSION@
+Cflags: -I${includedir}
+Libs: -L${libdir} -lspot
+Requires: libbddx