The MolScript software can be obtained from its GitHub repository.
Although GNU's bison (GNU's equivalent of yacc) was used to develop the program, it is not required to compile the program, since the C source code files produced by bison from the molscript.y file are included in the distribution.
MolScript will compile with just the code that is included in the distribution. This gives the basic implementation which can output PostScript, Raster3D and VRML 2.0 files.
If your computer systems supports OpenGL rendering, then an implementation allowing an interactive graphics window can be compiled. This also requires the GLUT library by Mark Kilgard. The OpenGL implementation allows several different image file output formats to be produced. Encapsulated PostScript Files (EPS) and SGI (aka RGB) image files are supported without any additional software. The JPEG, PNG and GIF image file formats require the corresponding libraries. None of these additional libraries are included in the MolScript distribution. You must fetch them and install them yourself.