Add USE_LAPACK as CMake option, with default true

[nmusolino]

The scs Makefile recognizes the compile flag USE_LAPACK, which controls whether LAPACK is linked in. But when scs is used through CMake (with add_subdirectory("scs")), it unconditionally uses LAPACK; there is no way to set USE_LAPACK=0.

This MR adds USE_LAPACK as a CMake variable, alongside other CMake variables like like DLONG and USE_OPENMP. This should allow CMake-based projects to control the USE_LAPACK compile option. The default value is true, of course, for backward compatibility.

[nmusolino]

FYI @bodono .

[bodono]

Thanks @nmusolino !