datetime.h defines like PM conflict with external libraries

Hi,

In my llvm jit work I'd to

#undef PM
/* include some llvm headers */
#define PM 1

because llvm has a number of functions which have an argument named PM.
Now that works, but it's fairly ugly. Perhaps it would be a good idea to
name these defines in a manner that's slightly less likely to conflict?

Alternatively we could use #pragma push_macro() around the includes, but
that'd be a new dependency.

Better ideas?

Greetings,

Andres Freund