Add support for unsupported systems

[rail5]

Bash++ supports any GNU/Linux system.

Portability of supershells

If compiling to target Bash version 5.3, supershells work on all platforms.

When targeting version <= 5.2, supershells are known to break on MacOS and Haiku. At the moment, they rely on the non-standardized /dev/fd interface. This should be made more portable

Notes

• The test suite requires GNU Grep + perl regexp support
• On MacOS, the format of the date command must also be modified in the makefile.

Untested Systems

• Any of the BSDs
• Solaris