sage: set CXXFLAGS to match CFLAGS in sage-env

pkgs/applications/science/math/sage/sage-env.nix

@@ -151,6 +151,7 @@ writeTextFile rec {
 
     # needed for cython
     export CC='${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc'
+    export CXX='${stdenv.cc}/bin/${stdenv.cc.targetPrefix}c++'
     # cython needs to find these libraries, otherwise will fail with `ld: cannot find -lflint` or similar
     export LDFLAGS='${
       lib.concatStringsSep " " (map (pkg: "-L${pkg}/lib") [
@@ -178,6 +179,7 @@ writeTextFile rec {
         mpfr.dev
       ])
     }'
+    export CXXFLAGS=$CFLAGS
 
     export SAGE_LIB='${sagelib}/${python3.sitePackages}'