Enable meta.cross for mpich related packages

pkgs/mpich/default.nix

@@ -45,10 +45,10 @@ in mpich.overrideAttrs (old: {
   ];
 
   preFixup = ''
-    sed -i 's:^CC=.*:CC=gcc:' $out/bin/mpicc
-    sed -i 's:^CXX=.*:CXX=g++:' $out/bin/mpicxx
+    sed -i 's#^CC=.*#CC=''${CC:-gcc}#' $out/bin/mpicc
+    sed -i 's#^CXX=.*#CXX=''${CXX:-g++}#' $out/bin/mpicxx
   '' + lib.optionalString enableFortran ''
-    sed -i 's:^FC=.*:FC=gfortran:' $out/bin/mpifort
+    sed -i 's#^FC=.*#FC=''${FC:-gfortran}#' $out/bin/mpifort
   '';
 
   hardeningDisable = [ "all" ];