isl: use tarball for stable version.

* Also add note about version detection of isl.
* Tighten gmp detection.

Formula/isl.rb

@@ -2,20 +2,32 @@ require 'formula'
 
 class Isl < Formula
   homepage 'http://freecode.com/projects/isl'
-  url 'http://repo.or.cz/r/isl.git', :tag => 'isl-0.12.1'
+  # Note: Always use tarball instead of git tag for stable version.
+  #
+  # Currently isl detects its version using source code directory name
+  # and update isl_version() function accordingly.  All other names will
+  # result in isl_version() function returning "UNKNOWN" and hence break
+  # package detection.
+  url 'http://isl.gforge.inria.fr/isl-0.12.1.tar.bz2'
+  sha1 'a54e80a32bc3e06327053d77d6a81516d4f4b21f'
 
   head 'http://repo.or.cz/r/isl.git'
 
+  depends_on 'gmp'
+
+  if build.head?
     depends_on :autoconf => :build
     depends_on :automake => :build
     depends_on :libtool => :build
-  depends_on 'gmp'
+  end
 
   def install
-    system "./autogen.sh"
+    system "./autogen.sh" if build.head?
     system "./configure", "--disable-dependency-tracking",
+                          "--disable-silent-rules",
                           "--prefix=#{prefix}",
-                          "--disable-silent-rules"
+                          "--with-gmp=system",
+                          "--with-gmp-prefix=#{Formula.factory("gmp").opt_prefix}"
     system "make"
     system "make", "install"
     (share/"gdb/auto-load").install Dir["#{lib}/*-gdb.py"]