Add line number information to more warning and error messages.
Detect #else and #endif without corresponing #ifdef/#ifndef as error.
Detect missing #endif at end of file and print warning but continue.
Support for #undef has been added to reverse the effect of a prior #define.
It is no error if the argument value has not been defined before.
These changes may cause error aborts on malformed input files (e.g. with
spurious #else or #endif), but no such errors exist in the calendar files
in the FreeBSD base system and the calendar-data port and all tests pass.
More tests will be added in a follow-up commit to detect regressions that
might affect the newly added features.
This commit ends a series of updates that enhance the pre-processor and
make it behave much more like prior versions of the calendar progarm that
called cpp to pre-process the data files.
MFC after: 3 days
Relnotes: yes
8e8c5da47f