configure: rewrote 3 ARG_ENABLEs to get them to behave properly
In addition to proper quoting for M4, the macro would not behave totally as
expected: for example, providing '--disable-modelock' would enable the
feature instead of keeping it disabled as the default.
The definitions now properly behave as expectable, so the user can
explicitly specify how he wants the feature even if it is the default; it
will also fail on improper use which is always a good idea to ease possible
future evolution.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>