Allow to use system libmsgpack and some improvement

Fedora Guidelines don't allow to use bundle library, see https://fedoraproject.org/wiki/Packaging:No_Bundled_Libraries Debian have the same Guideline

The first commit add the --with-libmsgpack option, which is noop for standard build.

Improvements, for reflection (function prototypes):

    $ php --re graphdat
  - Functions {
    Function [ <internal:graphdat> function graphdat_begin ] {
      - Parameters [1] {
        Parameter #0 [ <required> $name ]
      }
    }
    Function [ <internal:graphdat> function graphdat_end ] {
      - Parameters [1] {
        Parameter #0 [ <required> $name ]
      }
    }

Improvement for phpinfo

    $ php -i 
    graphdat => enabled
    graphdat extension version => 1.0.3
    msgpack headers version => 0.5.8
    msgpack library version => 0.5.8

headers version is version used at buildtime. library version is version used at runtime