Compile rtc_base_objc and rtc_api_objc for Mac

webrtc/supplement.gypi

 {
   'variables': {
     'variables': {
       'webrtc_root%': '<(DEPTH)/webrtc',
       # Override the default (10.6) in Chromium's build/common.gypi.
-      # Needed for ARC and libc++.
-      'mac_deployment_target%': '10.7',
+      # Needed for ARC, libc++, and nullability.
+      'mac_deployment_target%': '10.10',
       # Disable use of sysroot for Linux. It's enabled by default in Chromium,
       # but it currently lacks the libudev-dev package.
       # TODO(kjellander): Remove when crbug.com/561584 is fixed.
       'use_sysroot': 0,
     },
     'webrtc_root%': '<(webrtc_root)',
     'mac_deployment_target%': '<(mac_deployment_target)',
+    'mac_deployment_target_major%': 10,

[kjellander_webrtc, 2016/02/01 06:08:29]

I don't like adding these variables just for comparing versions, especially as
that check will break when mac_deployment_target_major (if ever) becomes 11 and
mac_deployment_target_minor is reset to 0.

Since doing greater than string comparison with mac_deployment_target works
equally well (although a bit ugly), I suggest we keep that (and only that)
variable instead.

I've been digging around but couldn't find anything similar in the Chromium
codebase. Maybe you can reach out to the Chrome on iOS team and ask what the
best practice is for conditions based on SDK level in GYP? (since their iOS code
is closed-source).

[kjellander_webrtc, 2016/02/01 06:20:19]

I found at least one greather than string comparison in the Chromium code base:
https://code.google.com/p/chromium/codesearch#chromium/src/testing/iossim/ios...
I also found that it's possible to query substrings like this:
https://code.google.com/p/chromium/codesearch#chromium/src/tools/gyp/test/win...
in GYP (which I wasn't aware of), which may be useful if we want to check for
>=10 minor version without introducing new GYP variables.

+    'mac_deployment_target_minor%': 10,
     'use_sysroot%': '<(use_sysroot)',
     'build_with_chromium': 0,
     'conditions': [
       ['OS=="ios"', {
         # Default to using BoringSSL on iOS.
         'use_openssl%': 1,
 
         # Set target_subarch for if not already set. This is needed because the
         # Chromium iOS toolchain relies on target_subarch being set.
         'conditions': [
@@ skipping 27 matching lines @@
             ],
             'sources': [
               '<(webrtc_root)/build/sanitizers/tsan_suppressions_webrtc.cc',
             ],
           }],
         ],
       }],
     ],
   },
 }