toolchain: make emscripten toolchain work with rules_foreign_cc (proxy-wasm#16)

Signed-off-by: Lizan Zhou <lizan@tetrate.io>

Author: lizan

toolchain/common.sh

@@ -20,16 +20,20 @@ if [[ "$OSTYPE" == "linux-gnu" ]]; then
 cd -P /proc/self/cwd
 fi
 
+TOOLCHAIN_ROOT=${EXT_BUILD_ROOT:-$PWD}
+
 export NODE_JS=''
-export EMSCRIPTEN_ROOT='external/emscripten_toolchain'
+export EMSCRIPTEN_ROOT="${TOOLCHAIN_ROOT}/external/emscripten_toolchain"
 export SPIDERMONKEY_ENGINE=''
 export EM_EXCLUSIVE_CACHE_ACCESS=1
 export EMCC_SKIP_SANITY_CHECK=1
 export EMCC_WASM_BACKEND=1
 
-source external/emscripten_toolchain/emsdk_env.sh
+#echo "$(pwd)/external/emscripten_toolchain/emsdk_env.sh"
+#realpath "external/emscripten_toolchain/emsdk_env.sh"
+source "${EMSCRIPTEN_ROOT}/emsdk_env.sh"
 
  # the emscripten sdk does some path comparison, so make EM_CACHE an absolute path to make it work. 
-mkdir -p "tmp/emscripten_cache"
-export EM_CACHE=${PWD}"/tmp/emscripten_cache"
-export TEMP_DIR="tmp"
+mkdir -p "${TOOLCHAIN_ROOT}/tmp/emscripten_cache"
+export EM_CACHE="${TOOLCHAIN_ROOT}/tmp/emscripten_cache"
+export TEMP_DIR="${TOOLCHAIN_ROOT}/tmp"