passthrough PLATFORM variable to sub builds

For tvos and other apple platforms, "PLATFORM" cmake variable
is set. We want to pass this through to sub-builds to build external
depenedencies.

Author: vimanyu

cmake/external_rules.cmake

@@ -66,6 +66,7 @@ function(download_external_sources)
       ${ENV_COMMAND} ${CMAKE_COMMAND}
       -G ${CMAKE_GENERATOR}
       -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}
+      -DPLATFORM=${PLATFORM}
       -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
       -DCMAKE_INSTALL_PREFIX=${FIREBASE_INSTALL_DIR}
       -DFIREBASE_DOWNLOAD_DIR=${FIREBASE_DOWNLOAD_DIR}