WAF quoting

[Sam Liddicott]

The WAF setup is also pretty rotten at quoting environment variables it 
repeats.

If I do:
export CPP="gcc -E"
so that CPP is (without quotes): gcc -E

then samba build fails because WAF invokes a line like this:
01:36:22 runner  cd .. && CPP=gcc -E ...  /usr/bin/perl 
".../samba.git/pidl/pidl" ... -- ".../samba.git/librpc/idl/atsvc.idl"
/bin/sh: -E: command not found

The expression of CPP=gcc -E on the command line should be quoted.

On the other hand, if I export:
CPP="'gcc -E'"
then it works fine - but fails with samba alpha11 that does things in a 
more normal way.

I've not managed to find any wscript file responsible for this so it may 
be a general WAF deficiency.

I've can't work round it by adding the -E to CPPFLAGS because that 
doesn't take effect.

I've worked around it in a way that works for me, but I thought this may 
be useful to whoever is looking at build-environment portability.

Sam