has_header result cached so can't add an include dir and provide the header for subprojects

[chrishecker]

I have a header file that is sometimes installed in the system and sometimes not. If I do a cc.has_header() check on it, and it returns false, I'd like to then add a global include directory (side comment: I wish there was a way to do this in a compiler independent manner using include_directory() instead of just a string) that will provide a version of the header to subprojects. This doesn't seem possible, because any later has_header() checks in the subprojects use the cached failure of the first call. Since they're subprojects I don't want to edit their build files to add the include there. Is there a way to tell meson to update the cache, or invalidate a cache entry?

Thanks,
Chris

[chrishecker]

Hmm, I tried to work around this by just doing a compiler version check at the very top, but it appears the global arguments aren't used in the has_header compilation? And, if I force it with include_directories on the has_header in the main meson.build, then it's not cached in the subproject because I assume the args are used as a key for the cache. So, I'm kinda stuck here, unless I modify the subproject...

Chris