/showIncludes log spam since Visual Studio 2026 version 18.7

[peterh]

Describe the bug
meson always adds /showIncludes when the compiler is MSVC, because ninja needs the output of /showIncludes to compute dependencies.

In previous Visual Studio versions, the /showIncludes flag in "Additional Options" in project files was always overridden by the XML version of this setting. In the most recent version of Visual Studio 2026, when the show includes setting is not set in the XML, it allows the Additional Options flag to apply.

This results in a lot of spam in the build log, leading to increased compile times. For example, in one of my larger projects a compile inside Visual Studio takes 10 minutes now, vs 7 minutes without the /showIncludes flag (vs 4 minutes with ninja).

To Reproduce
meson setup --backend=vs

Expected behavior
/showIncludes is only added when --backend=ninja

system parameters

• Is this a cross build or just a plain native build (for the same computer)? This is a native build using the vs2026 backend.
• what operating system? Windows 11
• what Python version are you using? The one bundled with the official meson-1.11.0-64.msi
• what meson --version? 1.11.0
• what ninja --version if it's a Ninja build? It's not a Ninja build.