Fix for non-portable GFortran -J flag in install script

[wyphan]

No description provided.

[awvwgk]

Can we use cd instead of pushd to avoid the issue with non-bash shells? Or if we want to stay with bash, why not just change the shebang to require a bash?

[wyphan]

Can we use cd instead of pushd to avoid the issue with non-bash shells?

I wrote a custom pushd and popd (without the stack) to deal with non-bash shells, and that's the part that I need extra testing...

Or if we want to stay with bash, why not just change the shebang to require a bash?

I honestly prefer portability so fpm can be installed anywhere, even on minimalist distros like Alpine or TinyCore. This would be beneficial for Docker usage.

[awvwgk]

Only one minor comment left, otherwise looks good to me.