[patch, docs] install.texi tweaks for gmp/mpfr options

[Brian Dessent]

This patch attempts to address the following shortcomings in
install.texi:

- The 'Prerequisites' section only gives a link to the MPFR site, but
not to the GMP site.

- The 'Prerequisites' section tries to mention the various
--with-{gmp,mpfr}{,-lib,-include} options, but this is duplicating an
existing section under 'Configuration'.

- It's not made clear that 4.3 is the first version where gmp/mpfr are
required universally, and that 4.2 only required them if gfortran was
enabled.  Note that while the 4.3 requirement is in fact already
mentioned in <http://gcc.gnu.org/gcc-4.3/changes.html>, the 4.2 gfortran
requirement  is *not* mentioned in
<http://gcc.gnu.org/gcc-4.2/changes.html>.  Moreover, it's reasonable to
want to be able to point a user to the unversioned page
<http://gcc.gnu.org/install/prerequisites.html> and have the info there
make sense in the context of installing either 4.2 or 4.3.

- It's not explained anywhere that GMP and MPFR are host libraries, not
target libraries, and the resulting implication for building a cross. 
Also, a bootstrap failure due to installing shared GMP/MPFR to a
non-system location and then neglecting to adjust LD_LIBRARY_PATH seems
to come up fairly regularly in gcc-help@ postings and PRs so I feel it's
worth mentioning in the install instructions, even though this issue is
not specific to gcc or gmp/mpfr per se.

- The current wording of the --with-gmp/--with-mpfr section looks
atrocious when rendered as PDF, due to very awkward line length.

The patch addresses these as follows:

- Combine the two GMP and MPFR bullets into one in the prerequisites
section, since otherwise there's a lot of duplication in their
descriptions.

- Give a link/reference to the configure options section rather than
trying to explain the options in the prerequisite section.  I had to use
a small @ifhthml/@ifnothtml hack here because of the fact that
install.texi is processed piecewise when generating HTML, so the @anchor
will be in a different file.  There might be a cleaner way to do this,
I'm not a texinfo expert.

- Reword the description of how --with-gmp is shorthand for
--with-gmp-{lib,include} so as to be easier to read and look better in
the printed version.

- Add an explanation of the fact that these are host libraries, and
about coping with shared GMP/MPFR in non-default locations.

- Removed a couple instaces of trailing whitespace.

Tested with "make info" and "make pdf", and running install.texi2html.

Brian
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