Improved Makefile

[criadoperez]

Two improvements on Makefile:

• It will first check if required dependency g++ is installed. If not, inform the user
• Added a make help command to list possible options

Note that default behavior of command make has not changed, and will still default to make all, instead of the more common make help. This is done on purpose not to change the behavior of the command.

[romkatv]

Thanks!

I merged make help but not the explicit check for $(CXX). I find the original (without the explicit check) error message and the error code more appropriate and informative. Consistency is nice, too (there are no checks for mkdir or zsh).

[criadoperez]

Makes sense. Thanks.