devdraw: restore compilation on OS X 10.6

Also add some ignored files to .hgignore

R=rsc
http://codereview.appspot.com/6842089

.hgignore

@@ -18,3 +18,10 @@ y.tab.[ch]$
 ^dict/
 ^postscript/font/
 \.orig$
+config
+install.log
+install.sum
+last-change
+lib/codereview/codereview.pyc
+src/cmd/awk/y.output
+src/cmd/devdraw/latin1.h

CONTRIBUTORS

@@ -39,6 +39,7 @@ Rob Kroeger <robkroeger@gmail.com>
 Rob Pike <robpike@gmail.com>
 Russ Cox <rsc@swtch.com>
 Sean McKean <smckean83@gmail.com>
+Shenghou Ma <minux.ma@gmail.com>
 Ties Bos <gitbisector@gmail.com>
 Tim Newsham <tim.newsham@gmail.com>
 Tony Lainson <t.lainson@gmail.com>

src/cmd/devdraw/cocoa-screen.m

@@ -1408,9 +1408,11 @@ winsizepoints()
 static NSSize
 winsizepixels()
 {
+#if DOSX_VERSION >= 100700
 	if (OSX_VERSION >= 100700 && devdrawretina)
 		return [win.content convertSizeToBacking: winsizepoints()];
 	else
+#endif
 		return winsizepoints();
 }