configure.ac: Ensure libva is enabled before invoking pkg-config.
configure.ac: Ensure libva is enabled before invoking pkg-config.

PKG_CHECK_VAR can only check --variable=$NAME, so it cannot handle
modversion.

Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler