configure.ac: Don't search llvm-config if it's known

This way LLVM_CONFIG can bet set from an env variable if it's outside
the $llvm_prefix.

This is not a must, but it helps testing.

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