Fix GCC7 C++17 build

GCC did not support <filesystem> until GCC8.

Fixes #1635

PiperOrigin-RevId: 615051227
Change-Id: If7cd5802ead40805b1ff1c3bdfc10ba6d2858ef0

absl/hash/hash_test.cc

@@ -48,7 +48,7 @@
 #include "absl/types/optional.h"
 #include "absl/types/variant.h"
 
-#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+#ifdef ABSL_INTERNAL_STD_FILESYSTEM_PATH_HASH_AVAILABLE
 #include <filesystem>  // NOLINT
 #endif
 

absl/hash/internal/hash.h

@@ -24,6 +24,15 @@
 #include <TargetConditionals.h>
 #endif
 
+#include "absl/base/config.h"
+
+// For feature testing and determining which headers can be included.
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+#include <version>
+#else
+#include <ciso646>
+#endif
+
 #include <algorithm>
 #include <array>
 #include <bitset>
@@ -47,7 +56,6 @@
 #include <utility>
 #include <vector>
 
-#include "absl/base/config.h"
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/base/port.h"
 #include "absl/container/fixed_array.h"
@@ -61,7 +69,7 @@
 #include "absl/types/variant.h"
 #include "absl/utility/utility.h"
 
-#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L && \
+#if defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L && \
     !defined(_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
 #include <filesystem>  // NOLINT
 #endif