Makefile: extract script to generate gitweb.cgi

Makefile: extract script to generate gitweb.cgi

In order to generate "gitweb.cgi" we have to replace various different
placeholders. This is done ad-hoc and is thus not easily reusable across
different build systems.

Introduce a new GITWEB-BUILD-OPTIONS.in template that we populate at
configuration time with the expected options. This script is then used
as input for a new "generate-gitweb.sh" script that generates the final
"gitweb.cgi" file. While this requires us to repeat the options multiple
times, it is in line to how we generate other build options like our
GIT-BUILD-OPTIONS file.

While at it, refactor how we replace the GITWEB_PROJECT_MAXDEPTH. Even
though this variable is supposed to be an integer, the source file has
the value quoted. The quotes are eventually stripped via sed(1), which
replaces `"@GITWEB_PROJECT_MAXDEPTH@"` with the actual value, which is
rather nonsensical. This is made clearer by just dropping the quotes in
the source file.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>