meson: Fix winflexbison warnings
Undefine __STDC_VERSION__ for C files to avoid mismatch with C11/C17.
Define __STDC_VERSION__ for C++ files to use path.
Splitting change off of !6162 (merged).
Edited by James Park
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Undefine __STDC_VERSION__ for C files to avoid mismatch with C11/C17.
Define __STDC_VERSION__ for C++ files to use path.
Splitting change off of !6162 (merged).