Parallelize Fortran compilation better by first creating .mod files with --syntax-only

[sujona]

In Fortran, the dependencies are in principle such that a file needs the .mod files of all used modules. You can run gfortran with --syntax-only to get the .mod files without compiling. This needs to be done in the right order according to the dependencies, but it is very fast. After that I think all the sources can be compiled in parallel without regards to the dependencies.

If this works, it could speed up the compilation quite a bit. I often see a few dependent files basically compiling in serial in the later stages.

[dcbaker]

Would you be interested in testing #12539 or pointing me at the projects you're looking at so I can test them? That should allow much greater parallelism by telling Ninja which Fortran sources produce which module interfaces and then ordering each source compilation on it's actual dependencies instead of full target dependencies