configure: warn about shared_glapi & xlib-glx only when both are set
configure: warn about shared_glapi & xlib-glx only when both are set

Printing out the message when shared_glapi is disabled only leads to
confusion.

Cc: "10.5 10.6" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler