Feature podspec modules (#793)

Summary:
This PR adds support for using Yoga Swift dependents when Yoga is pulled as a static library.

Currently swift projects trying to pull static libraries are unable to import the module. The reason for that is because the `DEFINES_MODULES` build setting is set to`NO`.  If a Swift Framework is trying to `pod spec lint --use-libraries` with `Yoga` as a dependency, the validation will fail. With the `DEFINES_MODULE` enabled, the product will be treated as defining its own module. This enables automatic production of LLVM module map files when appropriate, and allows the product to be imported as a module.

A workaround to this issue would be passing the `:modular_headers` flag to the `Podfile`, but that would not fix the `pod spec lint` validation for framework/library dependencies, it would just allow consuming applications to build and run.

An example of this issue would be [SonarKit](https://github.com/facebook/Sonar/blob/master/iOS/SonarKit.podspec). `SonarKit` wasn't able to validate its podspec due to `YogaKit` (Swift Framework) depending on `Yoga` and SonarKit validating with the `--use-libraries` flag due to the c++ dependencies. We had to create a new version of [Yoga 1.9 podspec](https://github.com/facebook/Sonar/blob/master/Specs/Yoga/1.9/Yoga.podspec.json) and make sure to set the `DEFINES_MODULE` flag of the pod target. After that, we were able to `pod spec lint` `SonarKit.podspec` successfully.

After merging a new `tag` should be created and the new podspec should be pushed to cocoapods master repository.
Pull Request resolved: https://github.com/facebook/yoga/pull/793

Reviewed By: passy

Differential Revision: D8820126

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 98eae3c31ec67a03c0fe97e05ab9e79446fa9f78

Yoga.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
   spec.name = 'Yoga'
-  spec.version = '1.8.1'
+  spec.version = '1.9.0'
   spec.license =  { :type => 'MIT', :file => "LICENSE" }
   spec.homepage = 'https://yogalayout.com/'
   spec.documentation_url = 'https://yogalayout.com/docs'
@@ -11,11 +11,14 @@ Pod::Spec.new do |spec|
   spec.authors = 'Facebook'
   spec.source = {
     :git => 'https://github.com/facebook/yoga.git',
-    :tag => '1.8.0',
+    :tag => spec.version.to_s,
   }
   spec.platforms = { :ios => "8.0", :tvos => "10.0" }
   spec.module_name = 'yoga'
   spec.requires_arc = false
+  spec.pod_target_xcconfig = {
+    'DEFINES_MODULE' => 'YES'
+  }
   spec.compiler_flags = [
       '-fno-omit-frame-pointer',
       '-fexceptions',

YogaKit.podspec

@@ -1,6 +1,6 @@
 podspec = Pod::Spec.new do |spec|
   spec.name = 'YogaKit'
-  spec.version = '1.8.1'
+  spec.version = '1.9.0'
   spec.license =  { :type => 'MIT', :file => "LICENSE" }
   spec.homepage = 'https://facebook.github.io/yoga/'
   spec.documentation_url = 'https://facebook.github.io/yoga/docs/api/yogakit/'
@@ -11,14 +11,14 @@ podspec = Pod::Spec.new do |spec|
   spec.authors = 'Facebook'
   spec.source = {
     :git => 'https://github.com/facebook/yoga.git',
-    :tag => '1.7.0',
+    :tag => spec.version.to_s,
   }
 
   spec.platform = :ios
   spec.ios.deployment_target = '8.0'
   spec.ios.frameworks = 'UIKit'
 
-  spec.dependency 'Yoga', '~> 1.8.1'
+  spec.dependency 'Yoga', '~> 1.9'
   spec.source_files = 'YogaKit/Source/*.{h,m,swift}'
   spec.public_header_files = 'YogaKit/Source/{YGLayout,UIView+Yoga}.h'
   spec.private_header_files = 'YogaKit/Source/YGLayout+Private.h'