I found in all make warning / error messages, it uses a single back quote but then a single quote to highlight the user input.

For example, the error message of the undefined makefile name is like

Makefile `xxx' was not found.

At first I thought it was a typo, but it seems to me it is intended for all messages.

For other information, see for example:

• discussion of "backtick-apostrophe" notation
• Markus Kuhn's explanation of how the X Window System displayed fonts