configure with relative srcdir causing absolute pathname in gcc/Makefile

[Doug Evans]

Ian Lance Taylor writes:
 > You remember correctly.  That was in large part in those days of
 > yesteryear running pwd on an automounted partition would return
 > something goofy which would not work after the automounter had timed
 > out the partition mount.

It's still the case that a directory on one machine needn't
have the same path when used on a different machine.