Creating LYX_DATE dynamically at configure-time caused unwanted recompilation
of the whole directories (src + src/tex2lyx) because all the relevant objects
were dependent of a common file (flags.make in case of "Unix makefiles") which changed
accordingly.
There is now a new include (lyx_date.h, with only one definition)
Nothing changes for automake, since in this case LYX_DATE is defined in config.h
