ext2fuse: fix build error by using a CFLAG

ext2fuse has been broken for a while because it's an old piece
of code that needs a few cflags for clang.  Macports also defines
a different precompiler variable, `-D__FreeBSD__=10` which it
turns out fixes all our build errors and aslo fixes the need we
had for defining `SSIZE_T`

- Add to CFLAGS `-D__FreeBSD__=10 -DENABLE_SWAPFS` like MP.
- Add a `--std=gnu89` flag if compiler is clang
- Remove the unneeded CFLAG.
- Works great with superenv.

Fixes Homebrew/homebrew#12207

Closes Homebrew/homebrew#15070.

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

Formula/ext2fuse.rb

@@ -10,7 +10,8 @@ class Ext2fuse < Formula
 
   def install
     ENV.append 'LIBS', "-lfuse4x"
-    ENV.append 'CPPFLAGS', "-DHAVE_TYPE_SSIZE_T=1"
+    ENV.append 'CFLAGS', '-D__FreeBSD__=10 -DENABLE_SWAPFS'
+    ENV.append 'CFLAGS', '--std=gnu89' if ENV.compiler == :clang
 
     system "./configure", "--disable-debug", "--disable-dependency-tracking",
                           "--prefix=#{prefix}"