Add patch for absl to ignore deprecated error.

cmake/external/abseil-cpp.cmake

@@ -31,4 +31,5 @@ ExternalProject_Add(
   INSTALL_COMMAND ""
   TEST_COMMAND ""
   HTTP_HEADER "${EXTERNAL_PROJECT_HTTP_HEADER}"
+  PATCH_COMMAND patch -Np1 -i ${CMAKE_CURRENT_LIST_DIR}/abseil-cpp.patch.txt
 )

cmake/external/abseil-cpp.patch.txt

@@ -0,0 +1,22 @@
+diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
+index d886cb30..9716c7b9 100644
+--- a/absl/meta/type_traits.h
++++ b/absl/meta/type_traits.h
+@@ -35,6 +35,10 @@
+ #ifndef ABSL_META_TYPE_TRAITS_H_
+ #define ABSL_META_TYPE_TRAITS_H_
+
++// Added by firestore-ios-sdk/cmake/external/abseil-cpp.patch.txt.
++#pragma clang diagnostic push
++#pragma clang diagnostic ignored "-Wdeprecated"
++
+ #include <cstddef>
+ #include <functional>
+ #include <type_traits>
+@@ -794,4 +798,6 @@ using swap_internal::StdSwapIsUnconstrained;
+ ABSL_NAMESPACE_END
+ }  // namespace absl
+
++#pragma clang diagnostic pop
++
+ #endif  // ABSL_META_TYPE_TRAITS_H_