Enable meta.cross for mpich related packages

pkgs/mpich/default.nix

@@ -45,11 +45,10 @@ in mpich.overrideAttrs (old: {
   ];
 
   preFixup = ''
-    # Ensure the default compilers are the ones mpich was built with
-    sed -i 's:CC="gcc":CC=${stdenv.cc}/bin/gcc:' $out/bin/mpicc
-    sed -i 's:CXX="g++":CXX=${stdenv.cc}/bin/g++:' $out/bin/mpicxx
+    sed -i 's:^CC=.*:CC=''${CC:-gcc}:' $out/bin/mpicc
+    sed -i 's:^CXX=.*:CXX=''${CXX:-g++}:' $out/bin/mpicxx
   '' + lib.optionalString enableFortran ''
-    sed -i 's:FC="gfortran":FC=${gfortran}/bin/gfortran:' $out/bin/mpifort
+    sed -i 's:^FC=.*:FC=''${FC:-gfortran}: $out/bin/mpifort
   '';
 
   hardeningDisable = [ "all" ];