INSTALL: Clarify --build, --host, --target, and the system types.

INSTALL: Clarify --build, --host, --target, and the system types.

* doc/install.texi (Compilers and Options): Add another reference.
(System Types): Renamed from System Type. Explain how to canonicalize
and how to validate a system type. Don't explain --build, --host,
--target here.
(Building for a different system type): New section.
(Troubleshooting the Build Type): New section.
(Configuring a Compiler): New section.
(configure Invocation): Mention the --host option, not the --build
option, since --build is so rarely needed.