shell function: confusing error when shebang incorrect

[Kirill Elagin]

Hi,

There is a bit of unexpected behaviour in the `shell` function (due to
the undocumented fact that it sometimes avoids actually calling the
shell):

```
$ cat Makefile
FOO:=$(shell ./foo.sh)

$ cat foo.sh
#!/bin/ohno
echo hi

$ make
make: ./foo.sh: No such file or directory
make: *** No targets.  Stop.

$ ./foo.sh
zsh: ./foo.sh: bad interpreter: /bin/ohno: no such file or directory
```

The “no such file or directory” error from Make is very confusing and
unexpected in this situation, especially given that it is not the
error that the shell would return.

The reason for it is that, while undocumented, the `shell` function
will try to avoid calling the shell in simple cases like this one and
will directly exec the command. However, the error returned by
`execve` is ambiguous:

> ENOENT  The file pathname or a script or ELF interpreter does not exist.

Shells (bash, zsh) disambiguate it themselves, i.e. there is extra
logic for the case of ENOENT, while Make simply fails with what it
sees, resulting in a puzzling error message.

Cheers,
Kirill