The preprocessor now compiles the MEX when use_dll is specified

New options "mexext" and "matlabroot" are introduced, so that the preprocessor
knows where to find MATLAB and which architecture to compile for.

Only recent gcc is now supported. A set of optimization flags is used so that
compilation goes reasonably fast on large models.

Consequently, options "msvc", "mingw" and "cygwin" have been removed.