Unused variable in release builds causes issues with `-Werror`

[jabraham17]

In a release build (i.e. no assert), there is an unused variable warning in syncvar.hpp

    int status = readFF(&ret);
    assert(status == QTHREAD_SUCCESS);

status is only used in the assert, so in a release build with -Werror this causes a build failure.

I found this when updating Chapel to qthreads 1.21, however I was able to workaround it by specifying -Wno-error=unused-variable.

[insertinterestingnamehere]

Thanks! Yah, sorry that one slipped through into the release. I'll get a fix up for it shortly.