enable build and run on FreeBSD

[tenomoto]

I made small changes to make the shell scripts run on FreeBSD and hopefully other BSD flaours.

• build.sh and run.sh do not use bash features. Replace /bin/bash with /bin/sh.
• MAKE is commonly used for make binary. Replace it with MAKEFILE.
• source/gfortran.makefile uses GNU extension of Make. Detect BSD and prepend g and set to MAKE.

[tenomoto]

I was able to hide a number of warning by setting -fmax-stack-var-size=524280. This is merely an ad hoc solution. It is better to allocate arrays in heap with allocate().

[tenomoto]

It turned out that setting -fmax-stack-var-size or -frecursive cause a runtime problem on macOS.

[samhatfield]

Hi Enomoto-san, thanks for your interest in speedy.f90. I welcome any and all suggestions for improvements. I have tried your branch on a Linux system and it builds and runs as expected.

I note your point on floating-point literals (#12). Indeed there are many implicit Float32 literals in the code which will be implicitly casted to Float64 when that is the working precision. For now though I would prefer not to implicitly convert untyped variables and literals to Float64 with -fdefault-real-8. In my opinion it's better to unambiguously specify the type when the variable is defined/literal is used, rather than rely on compiler behaviour. Could you revert that commit, and I can at some point go through the code and add _p to all literals?

[samhatfield]

Otherwise it looks like you've also implemented restart functionality (42b8171) which is great! Could you extract that commit and put it in a separate PR?