BLD: print long double format used

[mgorny]

Print the long double format that is used for the build. This can be helpful when you're about to cross-compile, but can also run Meson first on the native host to check the right value.

[seberg]

Looks fine to me. I guess it's already printed in the full log but rather cryptically. @ngoldbaum or is there a better way to do this?

I feel like there must be a place where all defines are printed already (so including the HAVE_... define), but I didn't find it immediately?

[rgommers]

LGTM, this is useful, thanks @mgorny. I've had this need as well before. We do the same with the symbol suffix for BLAS, which is printed as a message and is also needed sometimes as input to the build, just like this value is.

I feel like there must be a place where all defines are printed already (so including the HAVE_... define), but I didn't find it immediately?

You're thinking about the detailed logging in builddir/meson-logs/meson-log.txt, but that's not visible in CI and also gets cleaned up so it's a frequent point of confusion. The HAVE_ defines are different, since they're never needed as input to the build, just generated header values. And the checks for them are present in the main log, e.g. for HAVE_XLOCALE_H you see:

Has header "xlocale.h" : YES

The reason the long double format isn't visible is because it's not a standard header/function/attribute check but comes from parsing the string output of a meson.get_compiler('c').run invocation.

[mgorny]

Thanks!

And indeed, I was only able to find this in the list of defines, which implies access to the build tree and is nontrivial to read anyway. Logs are preserved more often than build trees, and having a single clear value makes it easy to copy and paste.