Make sure that math macros are updated at export time.

Make sure that math macros are updated at export time.

The math macros system is quite complex. Macros are updated during
metrics calculation, so a missing update is very likely to cause a
crash. This commit tries to assure that they are updated at export
time, which also happens when the table of contents is updated.
Moreover, in order to circumvent a possible missing update, when
a math macro is detected we try to avoid using the sym_ member
of the MacroData class, as it may contain bogus values.