libreoffice: fixup by building with gcc-5

I was lazy to try to find another solution.
/cc maintainers: @7c6f434c, @viric.

pkgs/top-level/all-packages.nix

···
     harfbuzz = harfbuzz.override {
       withIcu = true; withGraphite2 = true;
     };
+    # checking whether g++ supports C++14 or C++11... configure: error: no
+    stdenv = overrideCC stdenv gcc5;
   });
 
   libreoffice-still = lowPrio (callPackage ../applications/office/libreoffice/still.nix {
···
     harfbuzz = harfbuzz.override {
       withIcu = true; withGraphite2 = true;
     };
+    # checking whether g++ supports C++14 or C++11... configure: error: no
+    stdenv = overrideCC stdenv gcc5;
   });