libprotobuf-c.pc.in: Add URL, use standard macros

PACKAGE_DESCRIPTION is defined in configure.ac. PACKAGE_NAME,
PACKAGE_VERSION, and PACKAGE_URL come from AC_INIT.

This change will ensure that libprotobuf-c.pc.in could also be used by
CMake.

protobuf-c/libprotobuf-c.pc.in

@@ -4,9 +4,10 @@ libdir=@libdir@
 includedir=@includedir@
 bindir=@bindir@
 
-Name: libprotobuf-c
-Description: Protocol Buffers C library
-Version: @VERSION@
+Name: @PACKAGE_NAME@
+Description: @PACKAGE_DESCRIPTION@
+Version: @PACKAGE_VERSION@
+URL: @PACKAGE_URL@
 Libs: -L${libdir} -lprotobuf-c
 Libs.private:
 Cflags: -I${includedir}