configure.ac: add llvm_add_optional_component helper

We want to add "optional" components, which have been added with later
LLVM versions.

One such in-tree example is inteljitevents. Others are to follow
shortly.

v2: Use the correct function, add blank line between functions (Tobias)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
1 file changed