[3.1] gcc/doc/install.texi: "libf2c does not support shared library"

[Matthias Klose]

The description of the "--enable-shared" option reads:

 @samp{libf2c} and @samp{libiberty} do not support shared libraries at all.

However starting with 3.1, libf2c (g2c?) supports building a shared
library (at least on some platforms).