Fix a bunch of incorrect conversions from configure to Meson, which result
in different outcomes with --extra-cflags=-Werror.

pthread_setname_np needs "#define _GNU_SOURCE" on Linux (which I am using
also for the non-Linux check, so that it correctly fails with an error
about having too few parameters).

Fix struct checks to use has_type instead of has_symbol, and "#define
_GNU_SOURCE" too in the case of struct mmsghdr.

Remove an apostrophe that ended up at the end of a #include line.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>