build: use c11 everywhere

[ObserverOfTime]

This should allow core to use the same header while disabling it by default in scanners, though I haven't tested it.

I changed the name to TREE_SITTER_REUSE_ALLOCATOR to be more consistent with TREE_SITTER_FEATURE_WASM.

[maxbrunsfeld]

Do we need to require C11 mode? What newer C features are we using?

[amaanq]

Do we need to require C11 mode? What newer C features are we using?

anonymous unions, a bit more info here and here

[maxbrunsfeld]

anonymous unions

I see, thanks. Do you have a sense of how widely-supported the C11 standard is? What's the difference between gnu11 vs c11?

[ObserverOfTime]

According to Wikipedia it should be supported in every non-EOL system:

Some features of C11 are supported by the GCC starting with version 4.6,[16] Clang starting with version 3.1,[17] IBM XL C starting with version 12.1,[18] and Microsoft Visual C++ starting with VS 2019 (16.8)[19] in September 2020.

gnu11 enables GNU extensions.
The Makefile already used it so I added it everywhere in core.
Parsers/scanners likely won't need any extensions so I made them c11.