RFR: 7902970: Script build.sh will fail if the default shell is dash

[Guoxiong Li]

Hi all,

My local default shell is `dash`.


$ ls -al /bin/*sh*
-rwxr-xr-x 1 root root 1113504 Jun 7 2019 /bin/bash
-rwxr-xr-x 1 root root 121432 Jan 25 2018 /bin/dash
lrwxrwxrwx 1 root root 4 Sep 26 2020 /bin/rbash -> bash
lrwxrwxrwx 1 root root 4 Sep 26 2020 /bin/sh -> dash
lrwxrwxrwx 1 root root 4 Sep 26 2020 /bin/sh.distrib -> dash
lrwxrwxrwx 1 root root 7 Sep 18 2020 /bin/static-sh -> busybox


When I use the command `sh make/build.sh --help`, the build.sh will fail.
Because the `dash` doesn't support the variables such as `BASH_SOURCE`.

The successful command is `bash make/build.sh --help`, which uses `bash` explicitly
instead of the default shell `sh` implicitly.

This patch clarifies this situation in the README.
Thanks for taking the time to review.

Best Regards,
-- Guoxiong

-------------

Commit messages:
 - Fix README. Add the description about failing to build with the default shell

Changes: https://git.openjdk.java.net/jtreg/pull/12/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=12&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-7902970
  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/12.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/12/head:pull/12

PR: https://git.openjdk.java.net/jtreg/pull/12