CMake: try to clarify that Git version match failure is not an error.

The wording was so insufficient that it made people think it's a fatal
error, and subsequently made them suspicious because it seemed like the
fatal error is ignored.

src/Magnum/CMakeLists.txt

@@ -77,9 +77,9 @@ if(Git_FOUND)
         set(MAGNUM_VERSION_COMMIT " ${MAGNUM_VERSION_COMMIT}")
         set(MAGNUM_VERSION_HASH ${CMAKE_MATCH_4})
     elseif(_MAGNUM_VERSION_RESULT EQUAL 0)
-        message(WARNING "Can't match Git version from ${MAGNUM_VERSION_STRING}")
+        message(WARNING "Can't match Git version from ${MAGNUM_VERSION_STRING}, generating a fallback version.h instead")
     else()
-        message(WARNING "Can't match Git version: ${_MAGNUM_VERSION_ERROR}")
+        message(WARNING "Can't match Git version, generating a fallback version.h instead: ${_MAGNUM_VERSION_ERROR}")
     endif()
 endif()
 configure_file(${CMAKE_CURRENT_SOURCE_DIR}/version.h.cmake