Add `solarized-broken-srgb` custom variable.

This allows real sRGB colors where it's supported and Gen RGB where it's not. Fixes issue #19.

Adapted from a patch by @hdhoang.

README.md

@@ -74,16 +74,17 @@ Advanced Configuration
 Solarized will work out of the box with just the instructions specified above
 but does include several variables that can be customized.
 
-    variable name           default   optional
-    -------------------------------------------
-    solarized-termcolors=   16    |   256
-    solarized-degrade   =   nil   |   t
-    solarized-bold      =   t     |   nil
-    solarized-underline =   t     |   nil
-    solarized-italic    =   t     |   nil
-    solarized-contrast  =   normal|   high, low
-    solarized-visibility=   normal|   high, low
-    -------------------------------------------
+    variable name            default   optional
+    --------------------------------------------
+    solarized-termcolors =   16    |   256
+    solarized-degrade    =   nil   |   t
+    solarized-bold       =   t     |   nil
+    solarized-underline  =   t     |   nil
+    solarized-italic     =   t     |   nil
+    solarized-contrast   =   normal|   high, low
+    solarized-visibility =   normal|   high, low
+    solarized-broken-srgb=   nil   |   t (see details for Mac behavior)
+    --------------------------------------------
 
 ### Option Details
 
@@ -121,6 +122,14 @@ but does include several variables that can be customized.
     Special characters such as trailing whitespace, tabs, newlines, when
     displayed using `:set list` can be set to one of three levels depending on 
     your needs. Default value is `normal` with `high` and `low` options.
+    
+*   solarized-broken-srgb
+
+    Emacs [bug #8402](http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8402)
+    results in incorrect color handling on Macs. If this is `t` (the default
+    on Macs), Solarized works around it with alternative colors. However,
+    these colors are not totally portable, so you may be able to edit the
+    "Gen RGB" column in `solarized-definitions.el` to improve them further.
 
 Code Notes
 ----------

solarized-definitions.el

@@ -44,6 +44,14 @@ in order to expand or compress the tonal range displayed."
   :options '(high normal low)
   :group 'solarized)
 
+(defcustom solarized-broken-srgb (if (eq system-type 'darwin) t nil)
+  "Emacs bug #8402 results in incorrect color handling on Macs. If this is t
+(the default on Macs), Solarized works around it with alternative colors.
+However, these colors are not totally portable, so you may be able to edit the
+\"Gen RGB\" column in solarized-definitions.el to improve them further."
+  :type 'boolean
+  :group 'solarized)
+
 ;; FIXME: The Generic RGB colors will actually vary from device to device, but
 ;;        hopefully these are closer to the intended colors than the sRGB values
 ;;        that Emacs seems to dislike
@@ -74,7 +82,7 @@ in order to expand or compress the tonal range displayed."
            (let ((index (if window-system
                             (if solarized-degrade
                                 3
-			      2)
+			      (if solarized-broken-srgb 2 1))
 			  (if (= solarized-termcolors 256)
 			      3
 			    4))))