gambit-scheme: warn about build time with Clang/LLVM-GCC

Closes Homebrew/homebrew#11498.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/gambit-scheme.rb

@@ -28,6 +28,14 @@ class GambitScheme < Formula
             "--enable-single-host"]
     args << "--enable-shared" if ARGV.include? '--enable-shared'
 
+    unless ENV.compiler == :gcc
+      opoo <<-EOS.undent
+        Compiling Gambit Scheme with Clang or LLVM-GCC takes a very long time.
+        If you have GCC, you can compile it much faster with:
+          brew install gambit-scheme --use-gcc
+        EOS
+    end
+
     system "./configure", *args
     system "make check" if ARGV.include? '--with-check'