fortune: fix Darwin build

pkgs/tools/misc/fortune/default.nix

···
   preConfigure = ''
     sed -i "s|/usr/|$out/|" Makefile 
   '';
+
+  preBuild = ''
+    makeFlagsArray=("CC=$CC" "REGEXDEFS=-DHAVE_REGEX_H -DPOSIX_REGEX" "LDFLAGS=")
+  '';
   
   postInstall = ''
     ln -s $out/games/fortune $out/bin/fortune