glib2 OSX: add try_compiler_option_strictly

In recent OSX, default compiler of Command Line Tools 4.0 or higher is clang.
So, '-Wout-of-line-declaration' causes flood of warnings as follows:

```
warning: unknown warning option '-Wunsafe-loop-optimizations'; did you
mean
      '-Wout-of-line-declaration'? [-Wunknown-warning-option]
```
glib2: add `-Werror` option when checking option
glib2: use added `-Werror` version try_compiler
Merge pull request #230 from cosmo0920/compiler-option-warning-using-clang

glib2: using "-Werror" to check whether it can use compiler option

Patch by cosmo. Thanks!!!
glib2: return boolean for checking_for

Because checking_for expects boolean.
gdk3: name code
gdk3: introduce Gdk::Keyval for key values

Gdk::Keyval::GDK_KEY_x style is deprecated. Use
Gdk::Keyval::KEY_x style instead.
gtk3: use Gdk::Keyval::KEY_x style
gdk3: remove unused file
gtk2 homebrew: fix package name

gtk2 ->
gtk+
   ^
gtk3 homebrew: fix homebrew package name

gtk3 ->
gtk+3
   ^^