[configure] Add --enable-debug option

No compiler optimization build is easy to debug with debugger. It is
useful for developers.

If --enable-debug option is used on configure, we can build Hatohol
with no compiler optimization. The default behavior isn't changed.
(= -O2 build)
Merge pull request #273 from project-hatohol/add-enable-debug-option

[configure] Add --enable-debug option