dev-libs/mtxclient: work around LTO build failure

Closes: https://bugs.gentoo.org/861731
Signed-off-by: Ronny (tastytea) Gutbrod <gentoo@tastytea.de>

dev-libs/mtxclient/mtxclient-0.8.0.ebuild

@@ -3,7 +3,7 @@
 
 EAPI=8
 
-inherit cmake
+inherit cmake flag-o-matic
 
 DESCRIPTION="Client API library for Matrix, built on top of Boost.Asio"
 HOMEPAGE="https://github.com/Nheko-Reborn/mtxclient"
@@ -33,6 +33,14 @@ PATCHES=(
 	"${FILESDIR}/0.6.0_remove_network_tests.patch"
 )
 
+src_prepare() {
+	if use test; then
+		filter-lto # bug #861731
+	fi
+
+	cmake_src_prepare
+}
+
 src_configure() {
 	local -a mycmakeargs=(
 		-DBUILD_LIB_TESTS="$(usex test)"