The package boca-web tried to modify Apache's configuration directory,
which is forbidden by the Debian Policy and triggers Lintian error
apache2-reverse-dependency-ships-file-in-not-allowed-directory.

This direct modification was also unnecessary, as it was redone by the
post installation maintainer script.

A clean rule was added to the Makefile, to remove compiled binaries.
This fixes Lintian's tag source-contains-prebuilt-binary.

The tools safeexec and boca-submit-run-root-wrapper are now built with
optimisations (gcc optimisation level 2).

Replace redundant recipe for tools/safeexec rule with automatic
variables:
* `$^` expands to the names of all rule's prerequistes
* `$@` expands to the name of the rule's target

Further information is available in
https://www.gnu.org/software/make/manual/make.html.

The package Makefile contained Apache HTTPD commands, and some of them
were repeated in the boca-web post installation maintainer script. The
commands were removed from the Makefile, as this requires having a valid
apache2 installation as a build dependency.

Generate debug symbols by adding the `-g` flag to the `gcc` command and
populate the dbgsym packages generated by the tooling with the required
debug symbols.

This fixes the following Lintian warnings:
* boca-autojudge-dbgsym: debug-file-with-no-debug-symbols
* boca-submission-tools-dbgsym: debug-file-with-no-debug-symbols