Fix './configure --without-system-libsafec' mistakenly enabling syste…

[deviance]

…m-installed libsafec

The third argument to AC_ARG_WITH() is "action-if-given" [0]. That means that, when calling ./configure --without-system-libsafec, the current code enables the option instead of disabling it.

Just provide an empty argument to keep the proper value.

[0] https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/autoconf.html#Package-Options