Hi -- I've just noticed that this change breaks compilation for me,
because this test incorrectly fails to set CONFIG_IOVEC on a system
where the header defines 'struct iovec'. This seems to be because
"struct iovec" isn't a valid thing to test with has_header_symbol,
because it provokes a compiler error from clang.
https://github.com/mesonbuild/meson/issues/1975 says that for gcc
it's actually going to be wrong the other way (always setting CONFIG_IOVEC
whether the system header has the struct or not), because "struct wombat;"
is syntactically OK as a*declaration*, not a use.
Maybe we can work around this by testing for the presence of something else,
eg IOV_MAX or the readv or writev functions ?