Convert install guide to markdown

Adds `make install-guide` to make both a plain-text INSTALL for the
tarball, and a monolithic HTML install guide for uploading to the
website. Intended for processing with pandoc, but I don't think
there's any pandoc-specific extensions used. Naturally, if the build
system doesn't have pandoc, then this make target is not available,
just like the other documentation features.

Added more description about various SIMD instruction sets,
portability, and how to use compilers and the GMX_SIMD setting.

Added summary of important CMake options for typical GROMACS
installations.

This now does some variable interpolation to keep accuracy up without
active maintenance, so setting some such variables gets changed a bit.

Mentioned more optional build components

Minor textual changes to the install guide.

Fixes #1440. Refs #1242

Change-Id: Iff8e165759fbf8cc5199c8447e912403b8a79a17