app-crypt/libb2: addUSE=native-cflags to control feature guessing

Package-Manager: Portage-2.3.13, Repoman-2.3.3

app-crypt/libb2/libb2-0.98.ebuild

@@ -13,7 +13,7 @@ SRC_URI="https://github.com/BLAKE2/libb2/archive/${GITHASH}.tar.gz -> ${P}.tar.g
 LICENSE="CC0-1.0"
 SLOT="0"
 KEYWORDS="~amd64 ~ppc64"
-IUSE="static"
+IUSE="static native-cflags"
 
 DEPEND=""
 RDEPEND="${DEPEND}"
@@ -28,12 +28,14 @@ src_prepare() {
 }
 
 src_configure() {
-	econf $(use_enable static)
+	econf \
+		$(use_enable static) \
+		$(use_enable native-cflags native)
 }
 
 src_compile() {
-	# respect our CFLAGS
-	emake CFLAGS="${CFLAGS}"
+	# respect our CFLAGS when native-cflags is not in effect
+	emake $(use native-cflags && echo no)CFLAGS="${CFLAGS}"
 }
 
 src_install() {

app-crypt/libb2/metadata.xml

@@ -10,4 +10,12 @@
 
 		Official implementations from BLAKE2 project.
 	</longdescription>
+	<use>
+		<flag name="native-cflags">
+		Use CFLAGS as determined by configure script.  These flags will
+		enable all CPU specific features it find available, to further
+		optimise the implementation.  This discards any existing CFLAGS
+		set in make.conf or similar.
+		</flag>
+	</use>
 </pkgmetadata>