building

[Jim Wilson]

There is a much simpler explanation for your problem.  Gcc uses the 
environment variable $CPP to run the preprocessor if it is set, 
otherwise it uses "gcc -E".  If you have this environment variable set 
to something which isn't a preprocessor, then you will get this 
configure error.  Get rid of the CPP environment variable, and gcc will 
build.

I can reproduce your problem if I do
CPP=gcc
export CPP
../gcc/configure

Perhaps we should add a configure check to verify that $CPP is actually 
a preprocessor.

Jim