Why /bin/sh doesn't like the line: if test "x$my_var" == "xyes"; then

[jerry M]

configure file got this line and it causes the message: test: xyes: unexpected operator
But removing spaces around == or replacing == with = makes it to work.

On Linux though this line works fine.

Why spaces around == would cause failure?
What
is the version of /bin/sh currently used in 7.2? Where is it taken
from? --version, -v, -version don't ever print version I guess due to
FreeBSD policy of not versioning individual utilities.