Test building all headers with wxNO_IMPLICIT_WXSTRING_ENCODING

Check that all our public headers compile with this macro defined and
that using a char string without specifying its expected encoding
results in the expected compilation failure in this case.

tests/allheaders.cpp

@@ -0,0 +1,30 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        tests/allheaders.cpp
+// Purpose:     Compilation test for all headers
+// Author:      Vadim Zeitlin, Arrigo Marchiori
+// Created:     2020-04-20
+// Copyright:   (c) 2010,2020 Vadim Zeitlin, Wlodzimierz Skiba, Arrigo Marchiori
+///////////////////////////////////////////////////////////////////////////////
+
+// ----------------------------------------------------------------------------
+// headers
+// ----------------------------------------------------------------------------
+
+#include "testprec.h"
+
+#ifdef __BORLANDC__
+    #pragma hdrstop
+#endif
+
+#include "allheaders.h"
+
+TEST_CASE("wxNO_IMPLICIT_WXSTRING_ENCODING", "[string]")
+{
+    wxString s = wxASCII_STR("Hello, ASCII");
+    REQUIRE(s == L"Hello, ASCII");
+#ifdef TEST_IMPLICIT_WXSTRING_ENCODING
+    // Compilation of this should fail, because the macro
+    // wxNO_IMPLICIT_WXSTRING_ENCODING must be set
+    s = "Hello, implicit encoding";
+#endif
+}