Avoid a empty library build failure on Apple platforms

https://github.com/abseil/abseil-cpp/issues/1465 reports that
some CMake builds on Apply platforms issue

```
warning: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: archive library: libabsl_bad_any_cast_impl.a the table of contents is empty (no object file members in the library define global symbols)
```

Our CMake build handles this problem for header-only libraries by not
building a library at all. For some libraries, for example our polyfills,
the library is only conditionally empty. In these libraries, I added
a single char variable on Apple platforms as a workaround.

I have been able to reproduce the warnings reported in
https://github.com/abseil/abseil-cpp/issues/1465, but they don't fail the
build for me. I don't see them any more after this change.

PiperOrigin-RevId: 595480705
Change-Id: Ie48637e84ebae2f2aea4e2de83b146f30f6a76b9

absl/debugging/internal/stack_consumption.cc

@@ -185,4 +185,22 @@ int GetSignalHandlerStackConsumption(void (*signal_handler)(int)) {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+extern const char kAvoidEmptyStackConsumptionLibraryWarning;
+const char kAvoidEmptyStackConsumptionLibraryWarning = 0;
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION

absl/log/log_entry.cc

@@ -25,5 +25,17 @@ constexpr int LogEntry::kNoVerbosityLevel;
 constexpr int LogEntry::kNoVerboseLevel;
 #endif
 
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace log_internal {
+extern const char kAvoidEmptyLogEntryLibraryWarning;
+const char kAvoidEmptyLogEntryLibraryWarning = 0;
+}  // namespace log_internal
+#endif  // __APPLE__
+
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/strings/string_view.cc

@@ -241,4 +241,22 @@ constexpr string_view::size_type string_view::kMaxSize;
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+extern const char kAvoidEmptyStringViewLibraryWarning;
+const char kAvoidEmptyStringViewLibraryWarning = 0;
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_STRING_VIEW

absl/types/bad_any_cast.cc

@@ -43,4 +43,22 @@ void ThrowBadAnyCast() {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+extern const char kAvoidEmptyBadAnyCastLibraryWarning;
+const char kAvoidEmptyBadAnyCastLibraryWarning = 0;
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_ANY

absl/types/bad_optional_access.cc

@@ -45,4 +45,22 @@ void throw_bad_optional_access() {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+extern const char kAvoidEmptyBadOptionalAccessLibraryWarning;
+const char kAvoidEmptyBadOptionalAccessLibraryWarning = 0;
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_OPTIONAL

absl/types/bad_variant_access.cc

@@ -61,4 +61,22 @@ void Rethrow() {
 ABSL_NAMESPACE_END
 }  // namespace absl
 
+#else
+
+// https://github.com/abseil/abseil-cpp/issues/1465
+// CMake builds on Apple platforms error when libraries are empty.
+// Our CMake configuration can avoid this error on header-only libraries,
+// but since this library is conditionally empty, including a single
+// variable is an easy workaround.
+#ifdef __APPLE__
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+extern const char kAvoidEmptyBadVariantAccessLibraryWarning;
+const char kAvoidEmptyBadVariantAccessLibraryWarning = 0;
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // __APPLE__
+
 #endif  // ABSL_USES_STD_VARIANT