cgl: Fix warnings -Wtautological-constant-out-of-range-compare

In macro cgl_config.m:ADD_ATTR(), cast the compared value to an int
before comparing it to WAFFLE_DONT_CARE (which is itself an int). This
fixes tautological comparisons when the compared value is a bool.

Fixes 2 warnings like the following:

  src/waffle/cgl/cgl_config.m:207:5: warning: comparison of constant
          'WAFFLE_DONT_CARE' (-1) with expression of type 'const bool'
          is always true [-Wtautological-constant-out-of-range-compare]
      ADD_ATTR(kCGLPFASampleBuffers,      attrs->sample_buffers);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  src/waffle/cgl/cgl_config.m:173:21: note: expanded from macro 'ADD_ATTR'
        if ((value) != WAFFLE_DONT_CARE) { \
            ~~~~~~~ ^  ~~~~~~~~~~~~~~~~

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
cgl: Gestalt is deprecated, stop using it

Don't use Gestalt() to get system version. Instead, get the
'ProductVersion' key from
'/System/Library/CoreServices/SystemVersion.plist'. This is the same fix
that Firefox used.

Fixes the following compilation warning:

  warning: 'Gestalt' is deprecated: first deprecated in OS X 10.8
     [-Wdeprecated-declarations]

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
cgl: Fix validation of system version

I failed at hex :(

For OS X 10.7, check that platform->system_version_full >= 0x0a07, not
0x1070. Ditto for 10.9.

Fixes creation of OpenGL 3.2 contexts. Tested on OS X 10.9 Mavericks.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>