sbcl: Fix failing install HEAD due to no version

The make script in sbcl gets a version string from either a
file named version.lisp-expr or a release tag in the git repo.
HEAD has neither so requires a step to create the file
with a dummy version string before the make script is run.

This commit creates that file when installing from HEAD.

Closes Homebrew/homebrew#26880.

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

Formula/sbcl.rb

@@ -77,6 +77,7 @@ class Sbcl < Formula
 
       cd buildpath do
         ENV['SBCL_ARCH'] = 'x86' if build.build_32_bit?
+        Pathname.new("version.lisp-expr").write('"1.0.99.999"') if build.head?
         system "./make.sh", "--prefix=#{prefix}", "--xc-host=#{xc_cmdline}"
       end
     end