va: fix --version-script detection for lld >= 17

When building libva with lld (the llvm-project linker), version 17 or
later, an error similar to the following is emitted when linking
libva.so:

    ld: error: va/libva.so.2.2000.0.p/va_compat.c.o: symbol
    vaCreateSurfaces@VA_API_0.32.0 has undefined version VA_API_0.32.0

The root cause is that lld 17 checks linker version scripts more
strictly by default, and emits an error when undefined symbols or
undefined versions are referenced.

Earlier in the build, it turns out that due to these lld errors, va's
meson.build fails to detect `--version-script` support:

    Checking if "-Wl,--version-script" : links: NO

This is because the small test program used by meson to check whether a
shared library can be linked with the `libva.syms` version script is
completely empty, and therefore the two symbols in the version script,
`vaCreateSurfaces_0_32_0` and `vaCreateSurfaces`, are undefined.

Fix the problem by providing placeholder definitions for these symbols
in the `code` argument to meson's `cc.links()` function. This ensures
that meson correctly detects `--version-script` support with lld version
17 or later, and makes it possible to link the libva shared library.

Signed-off-by: Dimitry Andric <dimitry@andric.com>

va/meson.build

@@ -60,7 +60,7 @@ libva_sym_arg = '-Wl,-version-script,' + '@0@/@1@'.format(meson.current_source_d
 
 libva_link_args = []
 libva_link_depends = []
-if cc.links('', name: '-Wl,--version-script', args: ['-shared', libva_sym_arg])
+if cc.links('void vaCreateSurfaces_0_32_0(void) {} void vaCreateSurfaces() {}', name: '-Wl,--version-script', args: ['-shared', libva_sym_arg])
   libva_link_args = libva_sym_arg
   libva_link_depends = libva_sym
 endif