Meson: Fix check_header() for readline and gssapi

Meson: Fix check_header() for readline and gssapi

Since f039c2244110, the minimum version of meson supported is 0.57.2,
meaning that it is possible to use the result of declare_dependency()
when checking for headers with check_header().  There were two TODOs for
readline and gssapi to change declare_dependency() after upgrading to at
least 0.57.0, which were not addressed yet.

While on it, this fixes a comment related to str.replace().  The
function has been introduced in meson 0.58.0, not 0.56.

Author: Andreas Karlsson <andreas@proxel.se>
Reviewed-by: Tristan Partin <tristan@partin.io>
Discussion: https://postgr.es/m/00cd2e0c-85df-4cf9-a889-125d85e66980@proxel.se