The epoll-shim dependency did not get detected on FreeBSD,
because of missing prefix and dependency arguments in has_function()
call in include/meson.build.
Also added the path of epoll-shim headers include directory
(/usr/local/include/libepoll-shim/) to the compiler include search
paths for proper detection and build.
Signed-off-by: b-aaz <b-aazbsd@proton.me>