buildutil/glibtests.m4: fix bashism

buildutil/glibtests.m4: fix bashism

configure scripts need to be runnable with a POSIX-compliant /bin/sh.

On many (but not all!) systems, /bin/sh is provided by Bash, so errors
like this aren't spotted. Notably Debian defaults to /bin/sh provided
by dash which doesn't tolerate such bashisms as '=='.

This retains compatibility with bash.

Fixes configure warnings/errors like:
```

checking whether to build static libraries... no
./configure: 14795: test: unexpected operator
```

Signed-off-by: Sam James <sam@gentoo.org>