Exclude MSVC workaround on clang and gcc (#1306)

* Exclude MSVC workaround on clang and gcc

Resolves #1304  
On Windows, clang defines `_MSC_VER`, so the MSVC workaround needs to not happen when the clang define is set.


Signed-off-by: gregmarr <gregmarr@users.noreply.github.com>

* Update include/cpp2util.h

Co-authored-by: Matthieu HERNANDEZ <25429726+MatthieuHernandez@users.noreply.github.com>
Signed-off-by: Herb Sutter <herb.sutter@gmail.com>

---------

Signed-off-by: gregmarr <gregmarr@users.noreply.github.com>
Signed-off-by: Herb Sutter <herb.sutter@gmail.com>
Co-authored-by: Herb Sutter <herb.sutter@gmail.com>
Co-authored-by: Matthieu HERNANDEZ <25429726+MatthieuHernandez@users.noreply.github.com>

Author: 3 people

include/cpp2util.h

@@ -368,7 +368,7 @@ constexpr auto gcc_clang_msvc_min_versions(
 }
 
 
-#if defined(_MSC_VER)
+#if defined(_MSC_VER) && !defined(__clang_major__)
    // MSVC can't handle 'inline constexpr' variables yet in all cases
     #define CPP2_CONSTEXPR const
 #else