Update grass.rb

Upon testing the new commit, found an additional problem with r.external.
Adding 'GDAL_DYNAMIC=' to the make and make install steps fixes an error
with finding the gdal library for raster layers brought into grass with
r.external. See compile instructions that address this here:
http://gvsigce.sourceforge.net/wiki/index.php/Compiling_GRASS_GIS

Closes Homebrew/homebrew#24320.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/grass.rb

@@ -101,8 +101,8 @@ class Grass < Formula
     end
 
     system "./configure", "--prefix=#{prefix}", *args
-    system "make" # make and make install must be separate steps.
-    system "make install"
+    system "make GDAL_DYNAMIC=" # make and make install must be separate steps.
+    system "make GDAL_DYNAMIC= install" # GDAL_DYNAMIC set to blank for r.external compatability
   end
 
   def caveats