c_std=gnuXX,cXX needs future-feature warning

[bgilbert]

Describe the bug
Meson ≥ 1.3.0 warns if c_std=gnu99 (or any other gnuXX) is used with MSVC:

DEPRECATION: None of the values ['gnu99'] are supported by the c compiler.
However, the deprecated gnu99 std currently falls back to c99.
This will be an error in the future.
If the project supports both GNU and MSVC compilers, a value such as
"c_std=gnu11,c11" specifies that GNU is preferred but it can safely fallback to plain c11.

The phrase This will be an error in the future, and the corresponding hard failure with --fatal-meson-warnings, strongly encourages projects to move to c_std=gnu99,c99. However, the latter is a hard failure on Meson < 1.3.0:

meson.build:1:0: ERROR: Value "gnu99,c99" (of type "string") for combo option "C language standard to use" is not one of the choices. Possible choices are (as string): "none", "c89", "c99", "c11", "c17", "c18", "c2x", "gnu89", "gnu99", "gnu11", "gnu17", "gnu18", "gnu2x".

The workaround is to specify

default_options: [meson.version().version_compare('>=1.3.0') ? 'c_std=gnu99,c99' : 'c_std=gnu99']

in the project() call, but that's not at all obvious. Projects that don't CI with older Meson versions won't notice that they need this until a user tries to build with MSVC and an older Meson.

To Reproduce

project('c', 'c', default_options: 'c_std=gnu99,c99', meson_version: '>=1')

Expected behavior
Meson prints a warning that c_std=gnu99,c99 is not available before Meson 1.3.0.

system parameters

• Is this a cross build or just a plain native build (for the same computer)?
  native
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.)
  Fedora 40
• what Python version are you using e.g. 3.8.0
  3.12.5
• what meson --version
  1.5.0
• what ninja --version if it's a Ninja build
  n/a