Darwin: Avoid linker issues with pigweed clang

Author: ksperling-apple

build/config/compiler/BUILD.gn

@@ -14,6 +14,7 @@
 
 import("//build_overrides/build.gni")
 import("//build_overrides/pigweed.gni")
+import("//build_overrides/pigweed_environment.gni")
 import("${build_root}/chip/java/config.gni")
 import("${build_root}/config/compiler/compiler.gni")
 import("${build_root}/config/sysroot.gni")
@@ -348,7 +349,14 @@ config("cosmetic_default") {
 }
 
 config("runtime_default") {
-  if (is_clang) {  # Using Pigweed clang instead of Darwin host clang
+  if (is_clang &&
+      current_os == "mac") {  # Using Pigweed clang instead of Darwin host clang
+    # Without pw_env_setup_CIPD_PIGWEED defined the hostclang:no_system_libcpp
+    # config silently uses the system libc++, usually resulting in linker errors.
+    assert(
+        defined(pw_env_setup_CIPD_PIGWEED),
+        "//build_overrides/pigweed_environment.gni must define pw_env_setup_CIPD_PIGWEED when using pigweed clang")
+
     configs = [
       "$dir_pw_toolchain/host_clang:no_system_libcpp",
       "$dir_pw_toolchain/host_clang:xcode_sysroot",