LaTeX error when building a file from xindy documentation

[jfbu]

> 
> Which makes me suspect the explanation is that the Makefile
> does not use my default user shell bash, but the sh shell.

No, perhaps something having to do with this Makefile test



.PHONY: test

test:
	@echo $$POSIXLY_CORRECT


Then I get

$ make test
y

but when I execute directly in my bash shell.

$ echo $POSIXLY_CORRECT


I get nothing in output (blank line).

I will stop the spam here linking to this extensive discussion

https://unix.stackexchange.com/questions/65803/why-is-printf-better-than-echo/65819#65819

which however does not yet allow me to understand what is really happening