AIX build: xlc confusion

[Yotam Medini]

When building on AIX, without explicit setting of envirnment
variable CC=cc, the configure script picks (or _may_ pick) 'xlc'
which cause a failure when linking cccp.

This happens since in  egcs-970917/gcc/Makefile
  USE_ALLOCA= ` case "${CC}" in "${OLDCC}") echo "${ALLOCA}" ;; esac `
becomes:
  USE_ALLOCA= ` case "xlc" in "cc") echo alloca.o ;; esac `
which echos nothing.

-- yotam