ecl: use brewed gmp

Fixes Homebrew/legacy-homebrew#27929
Closes Homebrew/legacy-homebrew#28238

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

Formula/ecl.rb

@@ -5,14 +5,11 @@ class Ecl < Formula
   url 'https://downloads.sourceforge.net/project/ecls/ecls/13.5/ecl-13.5.1.tgz'
   sha1 'db7f732e5e12182118f00c02d8d2531f6d6aefb2'
 
-  fails_with :clang do
-    build 425
-    cause "The built-in gmp library fails to build with clang"
-  end
+  depends_on 'gmp'
 
   def install
     ENV.deparallelize
-    system "./configure", "--prefix=#{prefix}", "--enable-unicode"
+    system "./configure", "--prefix=#{prefix}", "--enable-unicode=yes", "--enable-threads=yes", "--with-system-gmp=yes"
     system "make"
     system "make install"
   end