When defining your own macros, you should now use
AC_DEFUN automatically calls
AC_PROVIDE and ensures that macros called via
do not interrupt other macros, to prevent nested ‘checking...’
messages on the screen. There's no actual harm in continuing to use the
older way, but it's less convenient and attractive. See Macro Definitions.
You probably looked at the macros that came with Autoconf as a guide for how to do things. It would be a good idea to take a look at the new versions of them, as the style is somewhat improved and they take advantage of some new features.
If you were doing tricky things with undocumented Autoconf internals (macros, variables, diversions), check whether you need to change anything to account for changes that have been made. Perhaps you can even use an officially supported technique in version 2 instead of kludging. Or perhaps not.
To speed up your locally written feature tests, add caching to them. See whether any of your tests are of general enough usefulness to encapsulate them into macros that you can share.