Also updates libcharset to the latest revision, which updates _its_
.gitignore (and pulls in other previous fixes too).

This commit updates the libcharset submodule to incorporate the
autotools-ification that I just pushed to that subproject, and builds
on it by replacing Halibut's own makefile system similarly with an
autotools setup.

The new Makefile.am incorporates both of the old Makefile and
doc/Makefile, so a single run of 'make' should now build Halibut
itself and all the formats of its own documentation, which also means
that the automake-generated 'make install' target can do the right
thing in terms of putting an appropriate subset of those documentation
formats in the assorted installation directories.

The old Makefiles are gone, as is release.sh (which is now obsolete
because autotools's 'make dist' doesn't do anything obviously wrong).
The bob build script is comprehensively rewritten, but should still
work - even the clang-based Windows build can use the
autotools-generated makefile system, provided I do the libcharset
build with a manual override of bin_PROGRAMS to prevent it trying to
build the libcharset supporting utilities (which are not completely
Windows-portable).

Now that I'm building a .chm as part of the default doc/Makefile
target, I should ignore it.

I often symlink it down from the build directory, so let's be
sympathetic to other people who like to type './halibut'.