Many of the macros were renamed in Autoconf version 2. You can still use the old names, but the new ones are clearer, and it's easier to find the documentation for them. See Obsolete Macros, for a table showing the new names for the old macros. Use the autoupdate program to convert your configure.ac to using the new macro names. See autoupdate Invocation.
Some macros have been superseded by similar ones that do the job better,
but are not call-compatible. If you get warnings about calling obsolete
macros while running autoconf, you may safely ignore them, but
your configure script generally works better if you follow
the advice that is printed about what to replace the obsolete macros with. In
particular, the mechanism for reporting the results of tests has
changed. If you were using echo or
AC_COMPILE_CHECK), your configure script's output
looks better if you switch to
AC_MSG_RESULT. See Printing Messages. Those macros work best
in conjunction with cache variables. See Caching Results.