games-emulation/dolphin: whitelist minizip, bug #671900

Richard reports build failure against live ebuild after
e6b2758ab4
upstream update:
  CMake Error at CMakeLists.txt:565 (add_subdirectory):
    add_subdirectory given source "Externals/minizip" which
    is not an existing directory.

Allow using stripped down minizip.

Reported-by: Richard Yao
Closes: https://bugs.gentoo.org/671900
Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

games-emulation/dolphin/dolphin-9999.ebuild

@@ -1,4 +1,4 @@
-# Copyright 1999-2018 Gentoo Foundation
+# Copyright 1999-2018 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 EAPI=6
@@ -79,6 +79,8 @@ src_prepare() {
 		glslang
 		# FIXME: xxhash can't be found by cmake
 		xxhash
+		# no support for for using system library
+		minizip
 		# soundtouch uses shorts, not floats
 		soundtouch
 		cubeb