build: adding CMAKE_TOOLCHAIN_FILE for source ExternalProject (#374)

Author: okhowang

CMakeLists.txt

@@ -55,6 +55,7 @@ ExternalProject_Add(BearSource
             -DCMAKE_CXX_FLAGS:STRING=${CMAKE_CXX_FLAGS}
             -DCMAKE_CXX_COMPILER:STRING=${CMAKE_CXX_COMPILER}
             -DPKG_CONFIG_EXECUTABLE:PATH=${PKG_CONFIG_EXECUTABLE}
+            -DCMAKE_TOOLCHAIN_FILE:PATH=${CMAKE_TOOLCHAIN_FILE}
         CMAKE_CACHE_ARGS
             -DCMAKE_PREFIX_PATH:PATH=${CMAKE_PREFIX_PATH}
             -DCMAKE_INSTALL_PREFIX:PATH=${STAGED_INSTALL_PREFIX}