Fix emsdk_env.sh for multiple shells (#594)

This change allows sourcing emsdk_env.sh from bash, zsh and
ksh.

The script works out the true location of the emsdk directory,
even if it is a symlink or the script itself is a symlink.

Added a test in scripts/test_source_env.sh to try sourcing via
all the shells and with various paths.

Co-authored-by: Bob Tolbert <bob@tolbert.org>

.circleci/config.yml

@@ -34,8 +34,9 @@ jobs:
       - checkout
       - run:
           name: Install debian packages
-          command: apt-get update -q && apt-get install -q -y cmake build-essential openjdk-8-jre-headless
+          command: apt-get update -q && apt-get install -q -y cmake build-essential openjdk-8-jre-headless ksh zsh
       - run: scripts/test.sh
+      - run: scripts/test_source_env.sh
       - run:
           name: test.py
           command: |