meson: simplify and parameterize various standard function checks

meson: simplify and parameterize various standard function checks

This is repetitive logic. We either want to use some -lc function, or if
it is not available we define it as -DNO_XXX and usually (but not
always) provide some custom compatibility impl instead.

Checking the intent of each block when reading through the file is slow
and not very DRY. Switch to taking an array of checkable functions
instead.

Not all functions are straightforward to move, since different macro
prefixes are used.

Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>