configure.ac: Fix --enable-thread-safety-constructor

Prior to this, --enable-thread-safety-constructor would disable it,
while --disable worked as expected, and no option left it enabled as
expected by default.  This also fixes the --help text to be displayed.

Fixes: afcdb6fb
Reported-by: @igor.v.kovalenko
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
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