build-sys: meson: Add some missing checks

- Header and function checks from configure.ac
  (some libc, some libpthread, dladdr from libdl)
- Find iconv and check constness
2 jobs for master in 12 minutes and 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #495287
build-autotools

00:12:03

passed #495288
build-meson

00:01:11