configure.ac: Add CFLAG -Wno-missing-field-initializers (v5)

configure.ac: Add CFLAG -Wno-missing-field-initializers (v5)

This warning is misleading: When a struct is partially initialized without
assigning to the structure members by name, then the remaining fields
will be zeroed out, and this warning will be issued (if enabled). If, on the
other hand, the partial initialization is done by assigning to named members,
the remaining structure elements may hold random data, but the warning is not
issued. Since in Mesa the first approach to initialize structure elements is
used very often, and it is usually assumed that the remaining elements are
zeroed out, heeding this warning would be counter-productive.

v2: - add -Wno-missing-field-initializers to meson-build
    - fix empty line error
    (both Eric Engestrom)

v3: * check for -Wmissing-field-initializers warning and then disable it
      because gcc and clang always accept -Wno-* (Dylan Baker)
    * Also disable this warning for C++

v4: * meson.build add -Wno-missing-field-initializers to
      c_args instead of no_override_init_args (Eric Engstrom)

v5: * configure.ac: Correct copy/paste error with CFLAGS/CXXFLAGS

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)
Reviewed-by: Emil Velikov <emil.velikov@collabora.com> (v2)
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>