Portable API for writing own macros

[Andrej Borsenkow]

Sometimes just the "official" macros, like AC_TRY_COMPILE etc are not
enough. 2.50 includes many very insteresting macros (AC_VAR_PUSHDEF/POPDEF,
AC_*_IFELSE, m4_* suite to name some). What macros can be "safel" used to
write own macros (safely meaning they remain in future versions)?

At the very least I'm interested in portable way to log output (bot to
stdout and into config.log) and/or redirect output.

TIA

-andrej