Add Gentoo Portage host configuration

env/compiler-clang.conf

@@ -0,0 +1,7 @@
+# /etc/portage/env/compiler-clang.conf
+CC="clang"
+CXX="clang++"
+CPP="clang-cpp"
+AR="llvm-ar"
+NM="llvm-nm"
+RANLIB="llvm-ranlib"