build: avoid error message during autogen.sh

Do not complain if the directory already exists.

mkdir: cannot create directory "autotools": File exists
build: fix not-honored --docdir setting

Despite using `./configure --docdir=XYZ` and/or `make docdir=XYZ`, the
directory is ignored. Resolve that problem.
build: do not create an INSTALL file

If the general-instruction INSTALL file is not desired, just specify so
in AUTOMAKE_OPTIONS rather than fiddling with it in autogen.sh.
build: kill redundant boilerplate in autogen.sh

The auxiliary build directory specified with AC_BUILD_AUX_DIR is
also automatically created AFAICS, so it need not be created
by autogen.sh.

[For directories that are not (such as with AC_CONFIG_MACRO_DIR),
place an empty .gitignore file in it, so that it always exists.]
build: adjust location of execgamesdir

For arch-dependent files, there is $exec_prefix. In most practical
cases, it has the same value as $prefix, but do use it anyhow.