Make podspec great again.

Summary:
Fixes #11272
Fixes #11572
Fixes #11781

The main changes here are:

* This depends on the latest CocoaPods (1.2.0). It’s currently in RC, but if I’m not mistaken a proper release is expected soon. /cc dantoml
* Adds required header search paths for the jschelpers and cxxreact subspecs.
* Makes the jschelpers and cxxreact headers private to building React Native, not visible to the user’s project.
* It uses the canonical upstream Yoga v1.0.0 podspec: https://github.com/facebook/yoga/blob/master/Yoga.podspec
* Consistent styling.

I have been able to get our app to build again using this https://github.com/artsy/emission/pull/437. The spec has some warnings, but otherwise fully passes lint.

rh389 sjmueller Could you please test with your projects?
Closes https://github.com/facebook/react-native/pull/12089

Differential Revision: D4518605

fbshipit-source-id: ecf86232d8b1af52d139eadd1acc10f5c1d42c29

ReactCommon/yoga/Yoga.podspec

@@ -0,0 +1,44 @@
+package = JSON.parse(File.read(File.expand_path('../../package.json', __dir__)))
+version = package['version']
+
+source = { :git => 'https://github.com/facebook/react-native.git' }
+if version == '1000.0.0'
+  # This is an unpublished version, use the latest commit hash of the react-native repo, which we’re presumably in.
+  source[:commit] = `git rev-parse HEAD`.strip
+else
+  source[:tag] = "v#{version}"
+end
+
+Pod::Spec.new do |spec|
+  spec.name = 'Yoga'
+  spec.version = "#{version}.React"
+  spec.license =  { :type => 'BSD' }
+  spec.homepage = 'https://facebook.github.io/yoga/'
+  spec.documentation_url = 'https://facebook.github.io/yoga/docs/api/c/'
+
+  spec.summary = 'Yoga is a cross-platform layout engine which implements Flexbox.'
+  spec.description = 'Yoga is a cross-platform layout engine enabling maximum collaboration within your team by implementing an API many designers are familiar with, and opening it up to developers across different platforms.'
+
+  spec.authors = 'Facebook'
+  spec.source = source
+
+  spec.module_name = 'yoga'
+  spec.requires_arc = false
+  spec.compiler_flags = [
+      '-fno-omit-frame-pointer',
+      '-fexceptions',
+      '-Wall',
+      '-Werror',
+      '-std=c11',
+      '-fPIC'
+  ]
+
+  # Pinning to the same version as React.podspec.
+  spec.platform = :ios, "8.0"
+
+  # Set this environment variable when not using the `:path` option to install the pod.
+  # E.g. when publishing this spec to a spec repo.
+  source_files = 'yoga/**/*.{c,h}'
+  source_files = File.join('ReactCommon/yoga', source_files) if ENV['INSTALL_YOGA_WITHOUT_PATH_OPTION']
+  spec.source_files = source_files
+end