[fpc-pascal] Exportable compile time variables/macros added to $mode MacPas

[Olle Raab]

Now macros/compile time variables defined in the interface part of a unit is
exported, in the same manner as ordinary pascal constructs, as in Metrowerks
pascal and Think Pascal.

Only a mode MacPas unit will export macros and only a mode MacPas unit will
import them. As before, MacPas and non-MacPas units can be used by each
other but macros will then not be visible for use.

Macros will start to be exported right after the {$MODE MACPAS} directive.
If no such directive is given, but -Mmacpas is given on the command line,
macros will start to be exported right after the UNIT keyword.

To support this, for a unit, {$MODE MACPAS} if forbidden after the UNIT
keyword. For other mode switches it is like before.

Caveat: If {$MODE MACPAS} is given before UNIT, the macros defined between
this point and UNIT is exported, if the unit is compiled as part of a
program or other unit, but is not exported if the unit is compiled from the
commandline (at top level). To be deterministic, do not put macros before
UNIT.

Olle