if Dynare++ will not be built, do not build mex files that depend on it
mex/build/matlab/configure.ac does not account for the case when makefiles are not created for dynare++ by the main configure script. Hence,
gensylv will still be built, causing an error. See #1482 (closed).
Easy solution: test for BLAS, LAPACK, and MatIO. don't build
dynare_simul_ if one of these libraries is not found (same test that we implement for building Dynare++ in the main config file.
Another solution would be to pass the value of