configure: fix broken bashisms resulting in logic failure

In a configure check that was carefully written for pre-unix-wars versions of the bourne shell, some code which was only valid using GNU bash was included.

Bash provides the standard test XXX = YYY or [ XXX = YYY ] utilities. It also provides the ability to spell the equals sign as a double equals. This does nothing whatsoever -- it adds no new functionality to bash, it forbids nothing, it is literally an exact alias.

It should never be used under any circumstances. All developers must immediately forget that it exists. Using it is non-portable and does not work in /bin/sh scripts such as configure scripts, and it results in dangerous muscle memory when used in bash scripts because it makes people unthinkingly use the double equals even in /bin/sh scripts. To add insult to injury, it makes scripts take up more disk space (by a whole byte! and sometimes even a few bytes...)

Delete this accidental bashism, and restore the ability to get correct ./configure behavior on systems where /bin/sh is something other than a symlink to GNU bash.