AM_ENABLE_MULTILIB: Allow switching the default

[Ralf Corsepius]

Hi,

another multilib related patch to allow switching the default of
AM_ENABLE_MULTILIB.

Background: The current default of AM_ENABLE_MULTILIB is to build
multilibs. This is OK for gcc/binutils/newlib etc., but is unsafe in
general otherwise, because config-ml.in internally assumes using gcc
with multilib support. 

I.e. building packages using AM_ENABLE_MULTILIB will almost certainly
fail to build if not using gcc. Furthermore, building with multilibs
enabled can dramatically increase compilation times (One gcc variant I
use applies ca. 30 multilib variants - 30 times the compilation time of 
non-multilib'ed builds)

I therefore prefer using a default of "no".

[May-be we should consider adding a check for "$(CC) -print-multi-lib"
and conditionally disable invoking config-ml.in based on this check.
Conditionalizing this calling config-ml.in based on test -n "$GCC" is
not enough, because older gccs do not support multilibs.]

Ralf

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