Passing --enable-doc=no or --disable-doc still enables the documentation

[orlitzky]

Given that AC_ARG_ENABLE is for boolean flags, its user interface doesn't match up with its intended purpose very well. Instead of two yes/no branches, the macro really gives you two present/absent branches, and you have to test the (boolean) value of the flag yourself in the "present" branch.

As a result, the --enable-doc flag doesn't work quite the way that you'd expect. In https://github.com/linbox-team/givaro/blob/master/macros/givaro-doc.m4#L39, that test enables the documentation whenever the --enable-doc flag is present. But the flag is "present" even when the user passes --disable-doc, or the equivalent --enable-doc=no. In those cases, the documentation should obviously not be built.