GNU makism crept in over weekend

[DJ Delorie]

> The following segment in a built gcc/Makefile is not
> handled by FreeBSD make:
> 
> gcov-iov.h: gcov-iov$(build_exeext)
>         ./$< > $@

Even in the GNU make documentation, most automatic variables are
documented in the section on pattern rules.  Their use in non-pattern
rules should be avoided (but let's fix them as we encounter them, so
that any that are supported by all the make variants we use will
continue to benefit the maintainers).  Please commit the obvious
patch.