Build system: by default, error out if some dependency is missing

In particular, if either MATLAB or Octave is missing, one needs to pass either
--disable-matlab or --disable-octave.

Moreover, several new configure flags have been introduced for disabling some
components:
--disable-doc
--disable-dynare++
--disable-mex-dynare++
--disable-mex-ms-sbvar
--disable-mex-kalman-steady-state