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.

debian/libbddx-dev.install

@@ -3,3 +3,4 @@ usr/include/bvecx.h
 usr/include/bddx.h
 usr/lib/*-*/libbddx.so
 usr/lib/*-*/libbddx.a
+usr/lib/*-*/pkgconfig/libbddx.pc

debian/libspot-dev.install

@@ -1,5 +1,7 @@
 usr/include/spot
 usr/lib/*-*/libspot.so
 usr/lib/*-*/libspot.a
+usr/lib/*-*/pkgconfig/libspot.pc
 usr/lib/*-*/libspotltsmin.so
 usr/lib/*-*/libspotltsmin.a
+usr/lib/*-*/pkgconfig/libspotltsmin.pc