Configure summary: autoconf essentially only parses configure.ac and
generates the configure script (that is, the "./configure" shell
script).  The latter is what actually checks what is available on the
system and internally sets the value of the output variables.  It then,
for every filename foo in AC_CONFIG_FILES (and for every output variable
name BAR in AC_SUBST), reads foo.in, replaces every occurrence of
`@BAR@` with the value of the shell variable `$BAR` and generates the
file foo from the result.  After this, configure is finished and `make`
could be executed to start the build.

Now that (as of netblue30#5140) all output variables are only defined on
config.mk.in and on config.sh.in, there is no need to generate any
makefile nor any other mkfile or shell script at configure time.  So
rename every "Makefile.in" to "Makefile", mkdeb.sh.in to mkdeb.sh,
src/common.mk.in to src/common.mk and leave just config.mk and config.sh
as the files to be generated at configure time.

This allows editing and committing all makefiles directly, without
potentially having to run ./configure in between.

Commands used to rename the makefiles:

    $ git ls-files -z -- '*Makefile.in' | xargs -0 -I '{}' sh -c \
      "git mv '{}' \"\$(dirname '{}')/Makefile\""

Additionally, from my (rudimentary) testing, this commit reduces the
time it takes to run ./configure by about 20~25% compared to commit
72ece92 ("Transmission fixes: drop private-lib (netblue30#5213)", 2022-06-22).
Environment: dash 0.5.11.5-1, gcc 12.1.0-2, Artix Linux, ext4 on an HDD.

Commands used for benchmarking each commit:

    $ : >time_configure && ./configure && make distclean &&
      for i in $(seq 1 10); do
      { time -p ./configure; } 2>>time_configure; done
    $ grep real time_configure |
      awk '{ total += $2 } END { print total/NR }'

This allows running `make clean` and `make distclean` (and possibly
others) without having to run ./configure beforehand.

Note that some packaging-related targets still depend on the existence
of generated files.  For example:

* dist: config.mk
* deb: config.sh

Commands used to search and replace:

    $ git grep -Elz 'include *([^ ]*/)?config.mk' | xargs -0 -I '{}' \
      sh -c "printf '%s\n' \
      \"\$(sed -E 's|^include *(([^ ]*/)?config.mk)|-include \1|' '{}')\" >'{}'"

Relates to netblue30#5140.

With the previous commit ("makefiles: stop failing when config.mk does
not exist", 2022-06-23), make will not immediately fail when trying to
build a target without having the proper compile-time flags (which are
defined on common.mk).

For example, when running the command below:

    make distclean && make

It will throw an error only after (mis-)compiling multiple objects.

So add a dependency on config.mk on every target that uses output
variables (such as @name@ / $(NAME)) on its recipe.  And add a
dependency on config.sh on targets that call shell scripts that use
output variables (such as @name@ / $NAME).  Also, add a recipe for
config.mk / config.sh telling to run ./configure, to make it a bit more
obvious just in case.

With this commit, make will abort earlier, by detecting that the
config.mk / config.sh dependency does not exist.  This happens before
trying to execute the recipe.

This also makes the dependencies more accurate, since if config.mk
(which defines some CFLAGS) is changed, the CFLAGS may also have
changed, so a target that uses CFLAGS should probably be considered out
of date in this case anyway.

Relates to netblue30#5140.