Refactor linux shared, stub config.

Refactor linux, stub platform config flags into 'default_cpp_standard'.

Only apply `default_cpp_standard` to default toolchain, avoid applying
it to other toolchains like host_toolchain.

Change-Id: Ia533601ceb7a0bd8f906dbb5ba885191ea4adb04

starboard/build/config/BUILD.gn

@@ -235,36 +235,48 @@ config("default_compiler_flags") {
 }
 
 # This config is intended to encapsulate all platform-specific logic for selecting the C++ standard
-# version in shared code. Starboard platform-specific build code should not attempt to overload this.
-# TODO: b/329683364 - Refactor the remaining related flags from each platforms platform_configuration to here.
+# version in shared code while building with the default toolchain.
+# Starboard platform-specific build code should not attempt to overload this.
 config("default_cpp_standard") {
-  if ((sb_is_modular && current_toolchain == cobalt_toolchain) || is_android) {
-    cflags_cc = [ "-std=c++17" ]
-  }
+  if (current_toolchain == default_toolchain) {
+    if (sb_is_modular || is_android) {
+      cflags_cc = [ "-std=c++17" ]
+    }
 
-  # These flags apply to non-modular windows platforms which includes win32, xb1.
-  if (is_host_win && !sb_is_modular) {
-    cflags = [ "/std:c++17" ]
-  }
+    # These flags apply to non-modular windows platforms which includes win32, xb1.
+    if (is_host_win && !sb_is_modular) {
+      cflags = [ "/std:c++17" ]
+    }
 
-  if (is_apple) {
-    cflags_cc = [ "-std=gnu++17" ]
-    cflags_objcc = [ "-std=gnu++17" ]
+    if (is_linux && !sb_is_modular) {
+      cflags_cc = [ "-std=gnu++17" ]
+    }
+
+    if (is_apple) {
+      cflags_cc = [ "-std=gnu++17" ]
+      cflags_objcc = [ "-std=gnu++17" ]
+    }
   }
 }
 
 config("cpp20_supported_config") {
-  if ((sb_is_modular && current_toolchain == cobalt_toolchain) || is_android) {
-    cflags_cc = [ "-std=c++20" ]
-  }
+  if (current_toolchain == default_toolchain) {
+    if (sb_is_modular || is_android) {
+      cflags_cc = [ "-std=c++20" ]
+    }
 
-  # These flags apply to non-modular windows platforms which includes win32, xb1.
-  if (is_host_win && !sb_is_modular) {
-    cflags = [ "/std:c++20" ]
-  }
+    # These flags apply to non-modular windows platforms which includes win32, xb1.
+    if (is_host_win && !sb_is_modular) {
+      cflags = [ "/std:c++20" ]
+    }
 
-  if (is_apple) {
-    cflags_cc = [ "-std=gnu++20" ]
-    cflags_objcc = [ "-std=gnu++20" ]
+    if (is_linux && !sb_is_modular) {
+      cflags_cc = [ "-std=gnu++20" ]
+    }
+
+    if (is_apple) {
+      cflags_cc = [ "-std=gnu++20" ]
+      cflags_objcc = [ "-std=gnu++20" ]
+    }
   }
 }

starboard/linux/shared/platform_configuration/BUILD.gn

@@ -98,7 +98,6 @@ config("compiler_flags") {
     "MESA_EGL_NO_X11_HEADERS",
   ]
   cflags_c += [ "-std=c11" ]
-  cflags_cc = [ "-std=gnu++17" ]
 
   if (use_asan) {
     cflags += [

starboard/stub/platform_configuration/BUILD.gn

@@ -16,7 +16,6 @@ config("platform_configuration") {
   configs = [ "//starboard/build/config/sabi" ]
   cflags = []
 
-  cflags_cc = [ "-std=gnu++17" ]
   ldflags = [ "-static-libstdc++" ]
 
   libs = [ "pthread" ]