Improve error message when a swift pod dep needs to be opted into defining a module

examples/Modules Example/CustomModuleMapPod/CustomModuleMapPod.podspec

@@ -14,8 +14,4 @@ Pod::Spec.new do |s|
 
   s.ios.deployment_target = '9.0'
   s.macos.deployment_target = '10.10'
-
-  s.pod_target_xcconfig = {
-    'DEFINES_MODULE' => 'YES',
-  }
 end

examples/Modules Example/ObjCPod/ObjCPod.podspec

@@ -13,8 +13,4 @@ Pod::Spec.new do |s|
 
   s.ios.deployment_target = '9.0'
   s.macos.deployment_target = '10.10'
-
-  s.pod_target_xcconfig = {
-    'DEFINES_MODULE' => 'YES',
-  }
 end

examples/Modules Example/Podfile

 workspace 'Examples.xcworkspace'
 
 abstract_target 'Abstract Target' do
+    use_modular_headers!
+
     pod 'ObjCPod', path: 'ObjCPod'
     pod 'SwiftPod', path: 'SwiftPod'
     pod 'MixedPod', path: 'MixedPod'

lib/cocoapods/installer/xcode/target_validator.rb

@@ -127,7 +127,8 @@ module Pod
                               'which do not define modules. ' \
                               'To opt into those targets generating module maps '\
                               '(which is necessary to import them from swift when building as static libraries), ' \
-                              "you may add `'DEFINES_MODULE' => 'YES'` to those pods' `pod_target_xcconfig`."
+                              'you may set `use_modular_header!` globally in your Podfile, '\
+                              'or specify `:modular_headers => true` for particular dependencies.'
           end
           return if error_messages.empty?
 

spec/unit/installer/xcode/target_validator_spec.rb

@@ -383,7 +383,7 @@ module Pod
             e.message.should.include <<-EOS.strip_heredoc.strip
               [!] The following swift pods cannot yet be integrated as static libraries:
 
-              The swift pod `OrangeFramework` depends upon `matryoshka`, which do not define modules. To opt into those targets generating module maps (which is necessary to import them from swift when building as static libraries), you may add `'DEFINES_MODULE' => 'YES'` to those pods' `pod_target_xcconfig`.
+              The swift pod `OrangeFramework` depends upon `matryoshka`, which do not define modules. To opt into those targets generating module maps (which is necessary to import them from swift when building as static libraries), you may set `use_modular_header!` globally in your Podfile, or specify `:modular_headers => true` for particular dependencies.
             EOS
           end