Add externConstexpr to MSVC compile options

We want this in order to be able to export constexpr members.

Change-Id: I33ba7964ebee54fe656df983985d8d6fa0b99358
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

cmake/QtInternalTargets.cmake

@@ -186,6 +186,11 @@ if (MSVC)
             -Zc:referenceBinding
         )
     endif()
+    if (MSVC_VERSION GREATER_EQUAL 1919)
+        target_compile_options(PlatformCommonInternal INTERFACE
+            -Zc:externConstexpr
+        )
+    endif()
 
     target_compile_options(PlatformCommonInternal INTERFACE -Zc:wchar_t)
 

mkspecs/common/msvc-version.conf

@@ -119,6 +119,7 @@ greaterThan(QMAKE_MSC_VER, 1919) {
     MSVC_VER            = 16.0
     QMAKE_CXXFLAGS_CXX2A = -std:c++latest
 
+    QMAKE_CXXFLAGS     += -Zc:externConstexpr
 }
 
 !isEmpty(COMPAT_MKSPEC):!$$COMPAT_MKSPEC: CONFIG += $$COMPAT_MKSPEC