RFC: Replace -isysroot with --sysroot?

[Hans-Peter Nilsson]

As it stands, -isysroot only affects header include search path.
The name is then right: for me, the leading "i" stands for
include path.  However, it's not really useful (at a glance at
least); you're likely to want the system path changed to that
same path as well.

So, can we add --sysroot, to affect the search paths as the
configure-option --with-sysroot=... does, but as a compile-time
option.

Is this acceptable for 4.0?

(Please, I do *not* want -isysroot to work as the universal
dynamic gcc-runtime sysroot option.)

brgds, H-P