Apply vlc_conf_prefix to all checks
Currently meson has no easy way to make our config.h stuff (like macros we set, and such) affect function checks, so what we are doing now is additionally write a large string into vlc_conf_prefix
which has all the #defines
too and add it to the prefix when checking for functions.
This is currently not done consistently though, but should be.
Eventually at some point meson will hopefully have a less ugly and error-prone way to do this.
(Not doing this causes some false-positives or false-negatives when checking for functions, as some macros like _GNU_SOURCE
affect function availability)