Don't generate sac.h, create it manually with appropriate ifdefs.

Currently, sac.h is being generated within the cmake call. The generation process simply takes all the runtime or libsac headers and puts #include <file> in sac.h.

The resulting file consists of ~100 includes which are traversed by a preprocessor every time we compile a sac program. Even thought this takes little time in the overall sac compilation, it becomes noticeable when using sac2c from jupyter notebooks, where we quickly compile small programs.

Therefore when compilation speed is important, it is better no to traverse all these files, so we include the headers conditionally.