misc: Create a pkg-config file

Make it more easy to find GNU lightning. Create and install
a pkg-config file.

.gitignore

@@ -30,3 +30,4 @@ m4/ltsugar.m4
 m4/ltversion.m4
 doc/mdate-sh
 doc/texinfo.tex
+lightning.pc

configure.ac

@@ -248,6 +248,7 @@ AC_SUBST(cpu)
 AC_SUBST([LIGHTNING_CFLAGS])
 
 AC_OUTPUT([Makefile
+	   lightning.pc
 	   doc/Makefile
 	   include/Makefile
 	   include/lightning/Makefile

lightning.pc.in

@@ -0,0 +1,12 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: GNU Lightning
+Description: JIT library
+Version: @VERSION@
+Libs: -L${libdir} -llightning
+Cflags: -I${includedir}/
+
+