external_rules.cmake: Pass along FIREBASE_PYTHON_HOST_EXECUTABLE because leveldb.cmake uses it (#9847)

Author: dconeybe

cmake/external_rules.cmake

@@ -27,6 +27,16 @@ function(download_external_sources)
     set(EXTERNAL_PROJECT_HTTP_HEADER "")
   endif()
 
+  # Pass along FIREBASE_PYTHON_HOST_EXECUTABLE because leveldb.cmake uses it.
+  if("${FIREBASE_PYTHON_HOST_EXECUTABLE}" STREQUAL "")
+    set(FIREBASE_PYTHON_HOST_EXECUTABLE_CMAKE_ARG "")
+  else()
+    set(
+      FIREBASE_PYTHON_HOST_EXECUTABLE_CMAKE_ARG
+      "-DFIREBASE_PYTHON_HOST_EXECUTABLE:FILEPATH=${FIREBASE_PYTHON_HOST_EXECUTABLE}"
+    )
+  endif()
+
   execute_process(
     COMMAND
       ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
@@ -36,6 +46,7 @@ function(download_external_sources)
       -DDOWNLOAD_BENCHMARK=${DOWNLOAD_BENCHMARK}
       -DDOWNLOAD_GOOGLETEST=${DOWNLOAD_GOOGLETEST}
       -DEXTERNAL_PROJECT_HTTP_HEADER=${EXTERNAL_PROJECT_HTTP_HEADER}
+      ${FIREBASE_PYTHON_HOST_EXECUTABLE_CMAKE_ARG}
       ${PROJECT_SOURCE_DIR}/cmake/external
     WORKING_DIRECTORY ${PROJECT_BINARY_DIR}/external
   )