More control over output file placement

[haasn]

Something unfortunate about the C generator (and probably others) is that the output files are always dumped into a fixed directory tree (include, src etc.).

Unfortunately, this makes it rather difficult to provide a meson wrapper for glad since, unlike the existing cmake wrapper, meson's equivalent (custom_target()) simply does not support generators that output files in a directory hierarchy - they have to all be output to the same directory for meson to pick them up as source files.

What I do currently is hack around it by moving the call to ./include/glad/meson.build and specifying --out-path=../.. which makes it end up dumping the header to the same directory as the meson.build - and thus able to be seen by custom_target(). But this has other annoying consequences, such as interfering with debugging (gdb doesn't pick up line information for functions inside the generated headers, for some reason), and also just making these files harder to inspect in general.

There are a number of improvements to be desired here, but I think the most direct solution to my problem would be some sort of --no-directories flag (or w/e) which would make it spit out a flat file hierarchy instead of grouping things into subdirectories.