add build option to use boost.locale instead of std::codecvt to compile on GCC < 5

[reefactor]

Hey, here is workaround for #12
Although it looks like a boilerplate of cmake for a small regression ^)

[reefactor]

I was unable to push branch directly to your repo:

Username for 'https://github.com': reefactor
Password for 'https://[email protected]': 
remote: Permission to bakwc/JamSpell.git denied to reefactor.
fatal: unable to access 'https://github.com/bakwc/JamSpell.git/': The requested URL returned error: 403

So I made this cross-fork-pull-request.
Can you add me to contributors?

[bakwc]

Thanks for you PR! There is no need to add to contributors for a single PR. I can add you if you plan to make more improvements.