Add mention of AC_CACHE_VAL

[rhelmot]

Hello! I'm trying to do a drive by contribution on an autoconf project in order to make it cross compile. I found this tutorial when looking for information on how to get past the fact that it is presently using a bare AC_RUN_IFELSE for a configuration check, and laughed out loud when I got to the part of the tutorial where it says, more or less, "don't do that :)".

I eventually found (by reading the bash source code of all things) that the right tool for the jobs is AC_CACHE_VAL. I think it would be nice if this tutorial mentioned that!

[edrosten]

I don't entirely follow. AC_CACHE_VAL retrieves something from the cache, AC_RUN_IFELSE checks if something ran. Are you referring to running the configure script in normal mode, then retrieving he cached value in cross compile mode?

I think I might be missing context on the specific problem here. Could you provide an example?

[rhelmot]

AC_CACHE_VAL retrieves a value from the cache (or command line!) if it exists and otherwise runs a clause to generate it. A way I've seen to make programs that use AC_RUN_IFELSE able to cross compile is to hardcode the cached value as determined by running the script on the target machine into the command line of the build script.