“Allow non-modular includes in framework modules”

It’s the magic sentence that allows pods dependent on RxLibrary to be
archived correctly by XCode.

It’s less than ideal, and seems arbitrary (why RxLibrary specifically?),
so we’ll try to produce a minimal case and open an issue with it in the
Cocoapods repo.
diff --git a/gRPC.podspec b/gRPC.podspec
index e5556cc..f95a6b3 100644
--- a/gRPC.podspec
+++ b/gRPC.podspec
@@ -65,4 +65,9 @@
 
   # Certificates, to be able to establish TLS connections:
   s.resource_bundles = { 'gRPCCertificates' => ['etc/roots.pem'] }
+
+  s.pod_target_xcconfig = {
+    # This is needed by all pods that depend on gRPC-RxLibrary:
+    'CLANG_ALLOW_NON_MODULAR_INCLUDES_IN_FRAMEWORK_MODULES' => 'YES',
+  }
 end