httpd: support groonga_query_log_path directive

Support query log with the same format as groonga server's one.

TODO: Document it.
Use subdir-objects option in Automake

automake 1.14 showes the following warning:

    vendor/mruby/sources.am:1: warning: source file '../mruby-4728070/src/vm.c' is in a subdirectory,
    vendor/mruby/sources.am:1: but option 'subdir-objects' is disabled
    vendor/mruby/Makefile.am:11:   'vendor/mruby/sources.am' included from here

So I use subdir-objects option.

See:
http://www.gnu.org/software/automake/manual/automake.html#index-subdir_002dobjects