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.pc.in

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