Fix building rules_foreign_cc targets in Bazel. (#968)

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

bazel/emscripten_toolchain/emar.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 
-source external/emsdk/emscripten_toolchain/env.sh
+source $(dirname $0)/env.sh
 
 exec python3 $EMSCRIPTEN/emar.py "$@"

bazel/emscripten_toolchain/emcc.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 
-source external/emsdk/emscripten_toolchain/env.sh
+source $(dirname $0)/env.sh
 
 exec python3 $EMSCRIPTEN/emcc.py "$@"

bazel/emscripten_toolchain/emcc_link.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 
-source external/emsdk/emscripten_toolchain/env.sh
+source $(dirname $0)/env.sh
 
-exec python3 external/emsdk/emscripten_toolchain/link_wrapper.py "$@"
+exec python3 $(dirname $0)/link_wrapper.py "$@"

bazel/emscripten_toolchain/env.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
 
-export ROOT_DIR=`(pwd -P)`
+export ROOT_DIR=${EXT_BUILD_ROOT:-$(pwd -P)}
 export EMSCRIPTEN=$ROOT_DIR/$EM_BIN_PATH/emscripten
 export EM_CONFIG=$ROOT_DIR/$EM_CONFIG_PATH