Modify install names for library and include files

Changing root name of library to json-c, and also the
directory where header files are installed to .../jsdon-c/*.

This avoids clashes with other implementations of JSON libraries.

json-c.pc.in

@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: json-c
+Description: JSON implementation in C
+Version: @VERSION@
+Requires: 
+Libs:  -L${libdir} -ljson-c
+Cflags: -I${includedir}/json-c