mariadb: Avoid building with clang-11

libcxx-11.1.0 uses `#include <version>` in several places. There's a
`storage/mroonga/version` file in the mariadb source and this gets
picked up by libcxx when building mroonga but that file doesn't contain
valid source code.

To avoid the issue altogether we're sticking with a clang-7 stdenv to
build mariadb for now.

Based on 787f934b247e41f7bb6d04959a66e0a111a0f0db from LnL7's WIP LLVM 9
update.

Co-authored-by: Daiderd Jordan <daiderd@gmail.com>

pkgs/top-level/all-packages.nix

@@ -21188,6 +21188,10 @@ with pkgs;
   };
 
   mariadb = callPackage ../servers/sql/mariadb {
+    # libcxx-11.1.0 includes `<version>` in some places and this picks up
+    # storage/mroonga/version under clang-11 so we build mariadb with a clang-7
+    # stdenv instead.
+    stdenv = if stdenv.cc.isClang then llvmPackages_7.stdenv else stdenv;
     inherit (darwin) cctools;
     inherit (darwin.apple_sdk.frameworks) CoreServices;
   };