codegen: Emit diagnostics pragmata for GCC 14, Clang 16 compatibility
GCC 14 refuses to compile C sources with certain type errors, to
provide prompt feedback to programmers about mistakes. Vala
does not generate valid C in some cases (see issues #358, #1005,
#1408, and perhaps others). As a stop-gap measure, this commit
introduces pragmata to turn the relevant GCC diagnostics into
warnings. For experimentation, this can be avoided at C compilation
time by defining the VALA_STRICT_C macro.
Clang 16 is broadly similar, except that it is not possible to
turn the errors into warnings using #pragma. They have to be disabled
completely. In Clang 16 and 17, -Wincompatible-pointer-types is not
an error yet, but that is likely to change in future Clang versions,
so this commit disables this warning, too. Another error,
-Wincompatible-function-pointer-types has been split from that,
and needs to be disabled. Clang does not yet have -Wreturn-mismatch,
but instead turns a subset of -Wreturn-type into errors, so that needs
to be disabled as well.
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