Adds support for compiling shared library for Luac

[guyzmo]

• compiles lua with .so shared lib per default
• added --no-shared to skip building shared lib (and avoid -fPIC addition)

Fixes #31

[ccinv]

I'm just a people passing by(in fact,a user of hererocks).I found your PR failed just because of pep8.
This patch is of great help.Could you fix it so that I can do CI with shared lua library soon?
Much Thanks. Ping @guyzmo.

[mpeterv]

@guyzmo @avaicode sorry for lack of review on this. @guyzmo I would prefer if this feature was opt-in, mostly because I want default behaviour to be as close to standard Lua makefile as possible.

[guyzmo]

fixed the pep8 and made it --shared.

[guyzmo]

i'm not gonna be around much more today, so feel free to make changes to the PR for inclusion 😉

[mpeterv]

@guyzmo I can not really get into this to fix the PR right now. If you could, please fix some issues:

• On Windows so_file is set to None but is used when building with MinGW, causing Windows tests to fail
• Name of the option was negated but behaviour wasnt
• On Mac Os X some of the used compiler options are not supported, see travis build