configure: Fix broken bashism resulting in logic failure (#567)

configure: Fix broken bashism resulting in logic failure (#567)

After carefully using configure checks designed to work on pre-unix wars
bourne shells -- that is, the `test "$var" = ""` construct once upon a
time wasn't specified to treat "" as a distinct argument, and various
buggy implementations mishandled various forms where the first argument
started with a dash, so the "x" padding provided a guaranteed comparison
-- the configure.ac check then fails to run on any shells at all other
than GNU bash.

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.