This document describes an approach applied to packaging zlib for build2. In
particular, this understanding will be useful when upgrading to a new upstream
version.

The upstream package contains the libz library, its usage examples, and tests.
Currently, we only package libz (see libz/README-DEV for details).

We add the upstream package as a git submodule and symlink the required files
and subdirectories into the build2 package subdirectories. Then, when required,
we "overlay" the upstream with our own headers, placing them into the library
directory.

Note that symlinking upstream submodule subdirectories into a build2 package
subdirectory results in creating intermediate build files (.d, .o, etc) inside
upstream directory while building the package in source tree. That's why we
need to make sure that packages do not share upstream source files via
subdirectory symlinks, not to also share the related intermediate files. If
several packages need to compile the same upstream source file, then only one
of them can symlink it via the parent directory while others must symlink it
directly. We also add the `ignore = untracked` configuration option into
.gitmodules to make sure that git ignores the intermediate build files under
upstream/ subdirectory.

The upstream package can be configured to contain a specific feature set. We
reproduce the union of features configured for the upstream source package in
Debian and Fedora distributions. The configuration options defining these sets
are specified in the Debian's rules and Fedora's RPM .spec files. These files
can be obtained as follows:

$ wget http://deb.debian.org/debian/pool/main/z/zlib/zlib_1.2.11.dfsg-1.debian.tar.xz
$ tar xf zlib_1.2.11.dfsg-1.debian.tar.xz debian/rules

$ wget https://kojipkgs.fedoraproject.org//packages/zlib/1.2.11/20.fc32/src/zlib-1.2.11-20.fc32.src.rpm
$ rpm2cpio zlib-1.2.11-20.fc32.src.rpm | cpio -civ '*.spec'

As a side note, on Debian and Fedora the source, libraries, and headers are
packaged as follows:

                src   library  headers
Debian/Ubuntu:  zlib  zlib1g   zlib1g-dev
Fedora/RHEL:    zlib  zlib     zlib-devel

Search for the Debian and Fedora packages at https://packages.debian.org/search
and https://apps.fedoraproject.org/packages/.

Both distributions use the default feature set and specify no additional
configuration options for the configure script.

Normally, when packaging a project, we need to replace some auto-generated
headers with our own implementations and deduce compilation/linking options.
For zlib we can rely for that on configure, Makefile.in, and
win32/Makefile.{gcc,msc}. In practice, however, that can be uneasy and error
prone, so you may also need to see the auto-generated files and the actual
compiler and linker command lines in the build log. If that's the case, you
can configure/build the upstream package on the platform of interest running
the following commands in the upstream project root directory.

On POSIX:

$ mkdir build
$ cd build
$ ../configure >build.log 2>&1
$ make >>build.log 2>&1

For MinGW GCC:

$ make -fwin32/Makefile.gcc >build.log 2>&1

For MSVC:

> nmake -f win32/Makefile.msc >build.log 2>&1

When the packaging is complete, build all the project packages in source tree
and make sure that no zlib headers are included from the system, running the
following commands from the project root:

$ cat `find . -name '*.d'` | sort -u >headers
$ emacs headers  # Edit, leaving system headers only.
$ fgrep zlib <headers