When libevhtp is added to a package's PKG_BUILD_DEPENDS (i.e. for
seafile-server), libevhtp's target package dependencies are not
automatically selected, in particular libevent2-openssl and
libevent2-pthreads.

Moreover, if libevent-openssl and libevent2-pthreads are not selected,
OpenSSL and thread support are disabled when compiling libevent2, which
in turn causes a compile error when building libevhtp.

This adds a config option, LIBEVHTP_BUILD_DEPENDS, that will select
libevent2-openssl and libevent2-pthreads (via LIBEVHTP_BUILD_SELECT)
when it is selected.

Other build dependencies are moved to PKG_BUILD_DEPENDS.

This also updates seafile-server to remove the (indirect) dependencies
added in 13d843fec1 and instead select
LIBEVHTP_BUILD_DEPENDS.

This also updates the maintainer's email address and adds myself as
another maintainer.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>