Makefile: Use standard features (IMPORTANT: default prefix changed)

Makefile: Use standard features (IMPORTANT: default prefix changed)

IMPORTANT for distributions:
This changes prefix to be '/usr/local' as is expected by default,
instead of the old '/usr' value.

- Use standard variables:
    - prefix should be '/usr/local'
    - mandir (instead of MANDIR)
    - htmldir (instead of HTDIR)
    - ...
    see <https://www.gnu.org/software/make/manual/html_node/Directory-Variables.html>

- Use standard targets:
    - html (build html files; don't install them)
    - install-html (instead of html)
    - installdirs (instead of 'mkdir -p'/'install -d' inside other targets)
    - ...
    see <https://www.gnu.org/software/make/manual/html_node/Standard-Targets.html#Standard-Targets>

- Use .PHONY

- ?= is not needed.  User input overrides any assignment.  Use =

- Use standard command variables, instead of directly calling commands.
    - $(INSTALL_DATA) (instead of install -m 644)
    - $(INSTALL_DIR) (instead of install -d -m 755 or mkdir -p)
    see <https://www.gnu.org/software/make/manual/html_node/Command-Variables.html#Command-Variables>

- Specify SHELL = /bin/bash

- Specify shebang

- Allow variable html extension (or no extension at all)

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>