Log Message:
libepoll-shim: take out CFLAGS from the pkg-config file

This is a patently Gross Hack<TM>.
But without it random package's configure scripts that use gtk3+ can pick
up broken epoll support via "pkg-config --cflags gtk+-x11-3.0", since that
will include -I/usr/pkg/include/libepoll-shim.

Direct consumers of libepoll-shim must add the CFLAGS explicitly.