Update macOS deployment target to 10.9.

Targeting 10.9 from a 10.10 bot is the configuration the NDK is
currently using.

It seems that to update to anything newer we probably need to change
how we're configuring binutils. I think this is related to the fact
that `xcode-select --install` no longer installs the sysroot to
/usr/local/include, and that's probably what our old GCC prebuilts are
expecting.

Bug: http://b/143105198
Test: None
Change-Id: I70aff99e82b5f856037d9859b74dbabe2fe4853f
Exempt-From-Owner-Approval: janitorial
diff --git a/do_build.py b/do_build.py
index 0d9b7d0..a23fa4f 100755
--- a/do_build.py
+++ b/do_build.py
@@ -105,7 +105,7 @@
         toolchain = ndk.paths.android_path(
             'prebuilts/gcc/darwin-x86/host/i686-apple-darwin-4.2.1')
         toolchain_prefix = 'i686-apple-darwin10'
-        env['MACOSX_DEPLOYMENT_TARGET'] = '10.8'
+        env['MACOSX_DEPLOYMENT_TARGET'] = '10.9'
     elif host == Host.Linux:
         toolchain = ndk.paths.android_path(
             'prebuilts/gcc/linux-x86/host/x86_64-linux-glibc2.15-4.8')