firebase-ios-sdk

Fix build under Linux/GCC Windows/MSVC (#3502)

* Avoid unconditionally setting C++ flags

These were already handled via CMAKE_C_FLAGS and CMAKE_CXX_FLAGS and
this breaks when compiling plain C under GCC, which considers unknown
options an error when compiling with -Werror.

* Fix compilation in Linux/GCC and also Windows/MSVC

GCC and MSVC aren't happy about `using Query = testutil::Query;` when
there's already `class Query` in scope.

* Fix objc_class.h in Windows

* Fix warnings that have crept in on Windows