Minor cleanup of Meson files given that we require 0.57

Hi,

I found a couple of comments about features not supported in meson 
versions previous to 0.57 but since we know do require 0.57.2 we can 
safely fix them.

The issue fixed is that check_header() previously did not support 
dependencies declared with declare_dependency() which seems like an easy 
fix which improves code quality a bit.

I also found a comment with a typo which claimed a feature requires 
0.56, but str.replace() actually requires 0.58.

-- 
Andreas Karlsson
Percona