MapGuide source code needs an MG_ASSERT macro

[waltweltonlair]

We current use a mix of _ASSERT(), assert(), and possibly others in our code. _ASSERT() works fine for Windows, but is not defined on Linux. assert() is defined on both Windows and Linux, but it will terminate the process if the assert fails (which is not the behavior we want).

We need to create an MG_ASSERT macro that checks the supplied condition only in Debug. If the check is false then a message box is displayed. Once closed the application continues running.