Show precise compiler version when running the unit tests

Although the compiler is already shown as part of the build signature,
it doesn't always show the exact compiler version used, which can be
useful as well, so add it here.

tests/test.cpp

@@ -541,6 +541,22 @@ bool TestApp::OnInit()
     cout << "Test program for wxWidgets non-GUI features\n"
 #endif
          << "build: " << WX_BUILD_OPTIONS_SIGNATURE << "\n"
+         << "compiled using "
+#if defined(__clang__)
+         << "clang " << __clang_major__ << "." << __clang_minor__ << "." << __clang_patchlevel__
+#elif defined(__INTEL_COMPILER)
+         << "icc " << __INTEL_COMPILER
+#elif defined(__GNUG__)
+         << "gcc " << __GNUC__ << "." << __GNUC_MINOR__
+#elif defined(_MSC_VER)
+         << "msvc " << _MSC_VER
+    #if defined(_MSC_FULL_VER)
+                                << " (full: " << _MSC_FULL_VER << ")"
+    #endif
+#else
+         << "unidentified compiler"
+#endif
+         << "\n"
          << "running under " << wxGetOsDescription()
          << " as " << wxGetUserId()
          << ", locale is " << setlocale(LC_ALL, NULL)