prerequisites.html

[Andrew Pinski]

I have been trying to fix the libcpp build failure and noticed
that I could not fix it as I do not have the software that is required.
Since nowhere is documented what is needed to autoconf/automake the 
libcpp
sub-directory.

Could someone who updated the sub-directories could please document the
requirements to rebuild the Makefile.in, aclocal.m4, and configure in
prerequisites.html.
Thanks,
Andrew Pinski