virtual/udev is an optional dependency, not required by disk/smart.

foo.diff

--- a/app-admin/collectd/collectd-5.5.1-r1.ebuild
+++ b/app-admin/collectd/collectd-5.5.1-r1.ebuild
@@ -16,7 +16,7 @@ SRC_URI="${HOMEPAGE}/files/${P}.tar.bz2"
 LICENSE="GPL-2"
 SLOT="0"
 KEYWORDS="~amd64 ~x86"
-IUSE="contrib debug java kernel_Darwin kernel_FreeBSD kernel_linux perl selinux static-libs"
+IUSE="contrib debug java kernel_Darwin kernel_FreeBSD kernel_linux perl selinux static-libs udev"

 # The plugin lists have to follow here since they extend IUSE

@@ -64,6 +64,7 @@ unset plugin
 COMMON_DEPEND="
 	dev-libs/libgcrypt:=
 	sys-devel/libtool
+	udev?					( virtual/udev )
 	perl?					( dev-lang/perl:=[ithreads] )
 	collectd_plugins_amqp?			( net-libs/rabbitmq-c )
 	collectd_plugins_apache?		( net-misc/curl )
@@ -74,7 +75,6 @@ COMMON_DEPEND="
 	collectd_plugins_curl_json?		( net-misc/curl dev-libs/yajl )
 	collectd_plugins_curl_xml?		( net-misc/curl dev-libs/libxml2:= )
 	collectd_plugins_dbi?			( dev-db/libdbi )
-	collectd_plugins_disk?			( virtual/udev )
 	collectd_plugins_dns?			( net-libs/libpcap )
 	collectd_plugins_genericjmx?		( virtual/jre:= dev-java/java-config-wrapper )
 	collectd_plugins_gmond?			( sys-cluster/ganglia )
@@ -104,7 +104,7 @@ COMMON_DEPEND="
 	collectd_plugins_rrdtool?		( net-analyzer/rrdtool )
 	collectd_plugins_sensors?		( sys-apps/lm_sensors )
 	collectd_plugins_sigrok?		( sci-libs/libsigrok )
-	collectd_plugins_smart?			( virtual/udev dev-libs/libatasmart )
+	collectd_plugins_smart?			( dev-libs/libatasmart )
 	collectd_plugins_snmp?			( net-analyzer/net-snmp )
 	collectd_plugins_tokyotyrant?		( net-misc/tokyotyrant )
 	collectd_plugins_varnish?		( www-servers/varnish )
@@ -316,6 +316,13 @@ src_configure() {
 	# Do we debug?
 	local myconf="$(use_enable debug)"

+	# udev support in disk/smart plugins?
+	if use udev; then
+		myconf+=" --with-libudev"
+	else
+		myconf+=" --without-libudev"
+	fi
+
 	local plugin

 	# Disable what needs to be disabled.