Makeconfig: Use $(error ...) to output error message

Being not familiar with the glibc repository, I typed "make" inside some
subdirectory that has a Makefile, and was met with:

    $ make
    ../Makeconfig:42: *** missing separator.  Stop.

At Makeconfig:42, we have:

    objdir must be defined by the build-directory Makefile.

Change that to use $(error ...), which I guess was the original
intention.  The result is:

    $ make
    ../Makeconfig:42: *** objdir must be defined by the build-directory Makefile.  Stop.