More conservative check for <string_view> availability. (google#4)

has_include(<string_view>) does not imply that the header can be
included and will work. The assumption fails on MSVC and libc++ [1, 2].
Conversely, checking that __cplusplus > 201402L is not sufficient on its
own either, as the toolchain on Mac OS 10.12 passes that check but does
not contain a <string_view> header.

[1] https://crbug.com/759349
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79433

include/crc32c/crc32c.h

@@ -33,22 +33,18 @@ inline uint32_t Crc32c(const std::string& string) {
                 string.size());
 }
 
-#if defined(__has_include)
+#if __cplusplus > 201402L
 #if __has_include(<string_view>)
-// Visual Studio provides a <string_view> header even in C++11 mode. When
-// included, the header issues an #error. (C1189)
-#if !defined(_MSC_VER) || __cplusplus >= 201703L
 #include <string_view>
 
-// Comptues the CRC32C of the bytes in the string_view.
+// Computes the CRC32C of the bytes in the string_view.
 inline uint32_t Crc32c(const std::string_view& string_view) {
   return Crc32c(reinterpret_cast<const uint8_t*>(string_view.data()),
                 string_view.size());
 }
 
-#endif  // !defined(_MSC_VER) || __cplusplus >= 201703L
 #endif  // __has_include(<string_view>)
-#endif  // defined(__has_include)
+#endif  // __cplusplus > 201402L
 
 }  // namespace crc32c
 

src/crc32c_unittest.cc

@@ -95,9 +95,8 @@ TEST(CRC32CTest, Crc32cStdString) {
   EXPECT_EQ(static_cast<uint32_t>(0x113fdb5c), crc32c::Crc32c(buf));
 }
 
-#if defined(__has_include)
+#if __cplusplus > 201402L
 #if __has_include(<string_view>)
-#if !defined(_MSC_VER) || __cplusplus >= 201703L
 
 TEST(CRC32CTest, Crc32cStdStringView) {
   uint8_t buf[32];
@@ -118,9 +117,8 @@ TEST(CRC32CTest, Crc32cStdStringView) {
   EXPECT_EQ(static_cast<uint32_t>(0x113fdb5c), crc32c::Crc32c(view));
 }
 
-#endif  // !defined(_MSC_VER) || __cplusplus >= 201703L
 #endif  // __has_include(<string_view>)
-#endif  // defined(__has_include)
+#endif  // __cplusplus > 201402L
 
 #define TESTED_EXTEND Extend
 #include "./crc32c_extend_unittests.h"