Obsolete building in source dir?

[DJ Delorie]

> If we print out 'creating build directory in <target>-build...
> configuring in <target>-build' then it's no-longer 'behind their back'.

Assuming the user catches that line in the heaping gobs of output
configure produces (especially when builds are automated).  Better to
say:

$ ./configure
configure: Please create a separate build directory for builds including gcc.

$

... so that the user KNOWS.

But, I'd prefer to just fix the bugs.  That's what we usually do (and
have historically done) when we find them.

I'm amenable to making $srcdir aboslute when it's relative, too.  That
should paper over a number of PRs in a relatively harmless way,
breaking only net installs where the mount points differ from machine
to machine (which is a bad practice anyway IMHO).