Add pkg-config file for libjq

pkg-config is a common interface to resolve build details. It will
become easier to incorporate libjq in other projects by providing this
interface.

libjq.pc.in

@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: libjq
+URL: https://stedolan.github.io/jq/
+Description: Library to process JSON using a query language
+Version: @VERSION@
+Libs: -L${libdir} -ljq
+Cflags: -I${includedir}