build: Use C89 for the standard version check
The '90' version is for ISO C90, but Meson does not understand it. As
far as GCC and Clang are concerned, -std=c89
is exactly the same as
-std=c90
. As of VS 2017, MSVC supports /std:c11
as the minimum
version of the C standard, with C89 (plus Microsoft extensions) being
the default.
See: