PR #1520: Make use of C++20 feature test macros to bestow better compatibility

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1520

Fixes #1519 and removes the burden of manually feature testing all the platforms supported by absl.
Merge 26bc4ee13a93edaeee2a21b5eff3569b806acc91 into 94b37802

Merging this change closes #1520

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1520 from georgthegreat:feature-test-macros 26bc4ee13a93edaeee2a21b5eff3569b806acc91
PiperOrigin-RevId: 558777775
Change-Id: Ie3e1f309761c67fa233a03e42762776cd93c69b4

absl/base/config.h

@@ -75,6 +75,12 @@
 #define ABSL_INTERNAL_CPLUSPLUS_LANG __cplusplus
 #endif
 
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+// Include library feature test macros.
+#include <version>
+#endif
+
 #if defined(__APPLE__)
 // Included for TARGET_OS_IPHONE, __IPHONE_OS_VERSION_MIN_REQUIRED,
 // __IPHONE_8_0.
@@ -557,6 +563,8 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 // Checks whether C++17 std::any is available.
 #ifdef ABSL_HAVE_STD_ANY
 #error "ABSL_HAVE_STD_ANY cannot be directly set."
+#elif defined(__cpp_lib_any) && __cpp_lib_any >= 201606L
+#define ABSL_HAVE_STD_ANY 1
 #elif defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
     ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L && \
     !ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE
@@ -568,6 +576,8 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 // Checks whether C++17 std::optional is available.
 #ifdef ABSL_HAVE_STD_OPTIONAL
 #error "ABSL_HAVE_STD_OPTIONAL cannot be directly set."
+#elif defined(__cpp_lib_optional) && __cpp_lib_optional >= 202106L
+#define ABSL_HAVE_STD_OPTIONAL 1
 #elif defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
     ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L && \
     !ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE
@@ -579,6 +589,8 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 // Checks whether C++17 std::variant is available.
 #ifdef ABSL_HAVE_STD_VARIANT
 #error "ABSL_HAVE_STD_VARIANT cannot be directly set."
+#elif defined(__cpp_lib_variant) && __cpp_lib_variant >= 201606L
+#define ABSL_HAVE_STD_VARIANT 1
 #elif defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
     ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L && \
     !ABSL_INTERNAL_APPLE_CXX17_TYPES_UNAVAILABLE
@@ -590,6 +602,8 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 // Checks whether C++17 std::string_view is available.
 #ifdef ABSL_HAVE_STD_STRING_VIEW
 #error "ABSL_HAVE_STD_STRING_VIEW cannot be directly set."
+#elif defined(__cpp_lib_string_view) && __cpp_lib_string_view >= 201606L
+#define ABSL_HAVE_STD_STRING_VIEW 1
 #elif defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
     ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
 #define ABSL_HAVE_STD_STRING_VIEW 1