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:

• GCC: https://gcc.gnu.org/onlinedocs/gcc-14.1.0/gcc/C-Dialect-Options.html
• MSVC: https://learn.microsoft.com/en-us/cpp/build/reference/std-specify-language-standard-version?view=msvc-170#c-standards-support-1