[gn build] Define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER when building l…

…ibcxx

Ports 87a8249 in a way. Has the effect of enabling warnings
when building libcxx itself, but more importantly this is required
after 5aaefa5 to not get build errors when building libcxx itself.

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -55,7 +55,10 @@ config("cxx_config") {
     "-std=c++20",
     "-nostdinc++",
   ]
-  defines = [ "_LIBCPP_BUILDING_LIBRARY" ]
+  defines = [
+    "_LIBCPP_BUILDING_LIBRARY",
+    "_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER",
+  ]
   if (target_os == "win") {
     cflags += [ "/Zl" ]
     defines += [