dev-util/source-highlight: fix build for Prefix, thanks Olivier Huber

Closes: https://bugs.gentoo.org/682330
Signed-off-by: Fabian Groffen <grobian@gentoo.org>
Package-Manager: Portage-2.3.62, Repoman-2.3.11

dev-util/source-highlight/source-highlight-3.1.8.ebuild

@@ -1,4 +1,4 @@
-# Copyright 1999-2018 Gentoo Foundation
+# Copyright 1999-2019 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 EAPI=6
@@ -23,6 +23,7 @@ src_configure() {
 	append-cxxflags -std=c++14
 
 	econf \
+		--with-boost="${EPREFIX}/usr" \
 		--with-boost-regex="boost_regex" \
 		--without-bash-completion \
 		$(use_enable static-libs static)