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.
diff --git a/configure.in b/configure.in
index 49b3195..df0d82b 100644
--- a/configure.in
+++ b/configure.in
@@ -31,5 +31,5 @@
 
 AC_OUTPUT([
 Makefile
-json.pc
+json-c.pc
 ])