ccls: fix libc++ header path

pkgs/development/tools/misc/ccls/default.nix

···
     cmakeFlagsArray+=(-DCMAKE_CXX_FLAGS="-fvisibility=hidden -fno-rtti")
   '';
 
+  clang = llvmPackages.clang;
   shell = runtimeShell;
-  postFixup = ''
-    # We need to tell ccls where to find the standard library headers.
 
-    standard_library_includes="\\\"-isystem\\\", \\\"${lib.getDev stdenv.cc.libc}/include\\\""
-    standard_library_includes+=", \\\"-isystem\\\", \\\"${llvmPackages.libcxx}/include/c++/v1\\\""
-    export standard_library_includes
-
-    wrapped=".ccls-wrapped"
-    export wrapped
-
+  postFixup = ''
+    export wrapped=".ccls-wrapped"
     mv $out/bin/ccls $out/bin/$wrapped
     substituteAll ${./wrapper} $out/bin/ccls
     chmod --reference=$out/bin/$wrapped $out/bin/ccls

pkgs/development/tools/misc/ccls/wrapper

···
 #! @shell@ -e
 
-initString="--init={\"clang\":{\"extraArgs\": [@standard_library_includes@"
-
-if [ "${NIX_CFLAGS_COMPILE}" != "" ]; then
-  read -a cflags_array <<< ${NIX_CFLAGS_COMPILE}
-  initString+=$(printf ', \"%s\"' "${cflags_array[@]}")
-fi
-
-initString+="]}}"
+printf -v extraArgs ',\"%s\"' \
+  $(cat @clang@/nix-support/libc-cflags \
+        @clang@/nix-support/libcxx-cxxflags) \
+  ${NIX_CFLAGS_COMPILE}
+initString="--init={\"clang\":{\"extraArgs\":[${extraArgs:1}]}}"
 
 exec -a "$0" "@out@/bin/@wrapped@" "${initString}" "$@"