[RFC/RFA] --with-build-time-tools configure option

[Paolo Bonzini]

>- I had to add an m4_pattern_allow to top level configure.in for
>AS_FOR_TARGET
>  
>
This is because you are using Autoconf 2.5x instead of 2.13.

>- I had to remove the ^ uses in the right hand operands of expr's :
>operator
>  
>
Didn't know about this non-portability.  Indeed expr checks only for 
anchored expressions, but I was trying to make it more clear.

>- Since you already introduced the use of
>ACX_CHECK_INSTALLED_TARGET_TOOL for as, ld, and nm, it seemed only
>logical to extend this to ar and ranlib
>  
>
I am not sure, because the purpose of this patch was to do the tests on 
the same tools that GCC will use after installation -- and GCC only uses 
as/ld/nm.  I think that if you want ar and ranlib placed somewhere else, 
you'd need to specify the new --with-build-time-tools options.

If we want to decide that $prefix/$target/bin is an interesting place to 
look for tools, it's fine by me but this should also apply to host 
tools, and should be coordinated with the Autoconf folks (which, sorry 
to say, means that likely it will never be done).

Paolo