cmake: find_package(cap) before linking against it

cmake: find_package(cap) before linking against it

before this change, we link against libcap without finding it. this
works fine as long as libcap-devel or libcap-dev is installed in the
system. but if it is not, the source would fail to build due to missing
`sys/capability.h`. this is not a great developer experience.

in this change, a `Findcap.cmake` is added to find the capability
library. which would fail the build at the configure phase.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>