make ./configure autoset gcc via CC & friends

[drewc]

The new %default-gerbil-gcc can be set! semi-permanently via the ./configure script by $CC or $GERBIL_GCC.

Then the gxc compiler (and bach) will always use that UNLESS it no longer exists or GERBIL_GCC is set, or CC is set.

If it cannot find them the old "use the first gcc in $PATH" applies. This is exported via (gerbil-gcc)

A config file will be added.

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[fare]

I really don't like the modifying a source file in-place. It doesn't play well with git.

Generating a different file, or having the file read some configuration at compile-time, would be much better.

[ober]

:+1: This bit me on a 5 hour build.