GRASS: Fix Lion builds

- Disable WxPython GUI. We have no stable WxMac build for this and building
    WxPython has the additional constraint of requiring a Framework build of
    Python.

  - Flag as failing with clang.

Fixes Homebrew/homebrew#8594.

Formula/grass.rb

@@ -8,6 +8,15 @@ def mysql?
     ARGV.include? "--with-mysql"
 end
 
+def headless?
+  # The GRASS GUI is based on WxPython. Unfortunately, Lion does not include
+  # this module so we have to drop it.
+  #
+  # This restriction can be lifted once WxMac hits a stable release that is
+  # 64-bit capable.
+  ARGV.include? '--without-gui' or MacOS.lion?
+end
+
 class Grass < Formula
   homepage 'http://grass.osgeo.org/'
   url 'http://grass.osgeo.org/grass64/source/grass-6.4.2.tar.gz'
@@ -28,10 +37,19 @@ class Grass < Formula
   # are not installed outside of the prefix.
   def patches; DATA; end
 
+  fails_with :clang do
+    build 318
+
+    cause <<-EOS.undent
+      Multiple build failures while compiling GRASS tools.
+      EOS
+  end
+
   def options
     [
       ['--with-postgres', 'Specify PostgreSQL as a dependency'],
-      ['--with-mysql', 'Specify MySQL as a dependency']
+      ['--with-mysql', 'Specify MySQL as a dependency'],
+      ['--without-gui', 'Build without WxPython interface. Command line tools still available.']
     ]
   end
 
@@ -50,7 +68,6 @@ class Grass < Formula
       "--with-x",
       "--without-motif",
       "--with-python=/usr/bin/python-config",
-      "--with-wxwidgets=/usr/bin/wx-config",
       "--with-blas",
       "--with-lapack",
       "--with-sqlite",
@@ -69,6 +86,12 @@ class Grass < Formula
       "--without-tcltk" # Disabled due to compatibility issues with OS X Tcl/Tk
     ]
 
+    if headless?
+      args << "--without-wxwidgets"
+    else
+      args << "--with-wxwidgets=/usr/bin/wx-config"
+    end
+
     if MacOS.prefer_64_bit?
       args << "--enable-64bit"
       args << "--with-macosx-archs=x86_64"
@@ -102,22 +125,33 @@ class Grass < Formula
   end
 
   def caveats
-    caveats = <<-EOS
-GRASS is currently in a transition period with respect to GUI support.  The old
-Tcl/Tk GUI cannot be built using the version of Tcl/Tk provided by OS X.  This
-has the unfortunate consquence of disabling the NVIZ visualization system.  A
-keg-only Tcl/Tk brew or some deep hackery of the GRASS source may be possible
-ways to get around this around this.
+    if headless?
+      <<-EOS.undent
+        This build of GRASS has been compiled without the WxPython GUI. This is
+        done by default on Lion because there is no stable build of WxPython
+        available to compile against.
 
-Tcl/Tk will eventually be depreciated in GRASS 7 and this version has been built
-to support the newer wxPython based GUI.  However, there is a problem as
-wxWidgets does not compile as a 64 bit library on OS X which affects Snow
-Leopard users.  In order to remedy this, the GRASS startup script:
+        The command line tools remain fully functional.
+        EOS
+    else
+      <<-EOS.undent
+        GRASS is currently in a transition period with respect to GUI support.
+        The old Tcl/Tk GUI cannot be built using the version of Tcl/Tk provided
+        by OS X.  This has the unfortunate consquence of disabling the NVIZ
+        visualization system.  A keg-only Tcl/Tk brew or some deep hackery of
+        the GRASS source may be possible ways to get around this around this.
 
-    #{prefix}/grass-6.4.0/etc/Init.sh
+        Tcl/Tk will eventually be depreciated in GRASS 7 and this version has
+        been built to support the newer wxPython based GUI.  However, there is
+        a problem as wxWidgets does not compile as a 64 bit library on OS X
+        which affects Snow Leopard users.  In order to remedy this, the GRASS
+        startup script:
 
-has been modified to use the OS X system Python and to start it in 32 bit mode.
-    EOS
+          #{prefix}/grass-6.4.0/etc/Init.sh
+
+        has been modified to use the OS X system Python and to start it in 32 bit mode.
+        EOS
+    end
   end
 end