sys-devel/llvm: Temporarily refuse to build w/ GCC-7 due to hangs

Bug: https://bugs.gentoo.org/649880

sys-devel/llvm/llvm-6.0.0.ebuild

@@ -75,6 +75,15 @@ S=${WORKDIR}/${P/_/}.src
 # least intrusive of all
 CMAKE_BUILD_TYPE=RelWithDebInfo
 
+pkg_pretend() {
+	if tc-is-gcc && [[ $(gcc-major-version) -ge 7 ]]; then
+		eerror "GCC 7 is known to cause mis-compilation that causes the build to hang."
+		eerror "Please use an older version to build LLVM until a good solution is found."
+		eerror "Bug report: https://bugs.gentoo.org/649880"
+		die "GCC-7+ is not supported"
+	fi
+}
+
 src_prepare() {
 	# Fix llvm-config for shared linking and sane flags
 	# https://bugs.gentoo.org/show_bug.cgi?id=565358