convert shell scripts to julia build.jl

[stevengj]

I'm not completely clear on how we want to build PETSc in the long run. (On Mac and Windows, we will probably need to ship dylib and dll binaries, respectively. On Ubuntu etc, we may want to use prepackaged PETSc binaries via bindeps.) But in the short run, where we are building from source, it would be more Julian to:

• Directly run the necessary download and make commands etc. from build.jl without using any shell script.
• Output a deps/deps.jl file containing constants const libpetsc_float64 = "...." etc. defining the paths of the library files (sans .so extension)

In particular, we shouldn't rely on the user (or Julia) setting any kind of PETSC_DIR environment variables if possible. Not only are those unfriendly to subprocesses, but they also don't seem like they will work for linking multiple Petsc versions simultaneously (#6).

[stevengj]

Closed by #19.