meson: use better zlib dependency fallback

zlib is required, and if it isn't found it is checked several ways and then forced via subproject(). This code was added in commit b93e3759, to account for systems where zlib doesn't have pkg-config files installed.

But Meson already does dependency fallback, and also, since 0.54.0, does the in-between checks for find_library('z') and has_header('zlib.h') via the "system" type dependency. Simplify dependency lookup by marking it as required, which also makes sure that the console log doesn't confusingly list "not found".