meson: fix readline detection if there is no pkg-config file

meson: fix readline detection if there is no pkg-config file

Commit <74416b1d4849ef77ef31de5344dd75f03094434b> added check for
rl_completion_quote_character to make sure we have correct readline
library. Commit <a9443bc9a9ef451b46306e66ed3b706756fc1414> added
inaccurate comment that it's a function.

We need to check for generic symbol instead of checking for function.
In addition the readline/readline.h file requires stdio.h to by included
beforehand which was done in autotools but I dropped it in meson.

And lastly the final condition to print error or disable readline was
broken as well by replacing the readline_dep every time if readline was
not explicitly enabled.

Reported-by: Erik Skultety <eskultet@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>