Build system improvements on Windows (MinGW / MSYS2)

- Use the -static flag when linking Dynare++, so that shipping libquadmath and
  libgcc DLL in the installer is no longer needed.

- Use AM_CXXFLAGS and AM_LDFLAGS variables for changing flags, since CXXFLAGS
  and LDFLAGS are user variables. Also, this avoids passing these flags down to
  configure scripts in subdirectories.

- Check for the SZIP library in the configure test for the MatIO, this is
  needed under MSYS2.

- Statically link MatIO and GSL in MEX files for MATLAB, this is needed under
  MSYS2.