Requirements:

 - Linux or mostly POSIX.1-2001 compliant environment
 - CMake 2.6.2 or newer, probably also GNU make
 - gcc 4.7 or newer or clang 3.0 or newer and latest libstdc++ or Boost headers
 - on Linux systems, kernel 2.6.22 or newer and glibc 2.8 or newer
 - pkg-config and zlib (and development components)

Extra requirements for optional features:
 - libbz2, liblzma (from XZ), OpenSSL and their development components
 - LibTomCrypt can serve as replacement for some parts of functionality if
   OpenSSL is not available. See LibTomCrypt notes below!
 - recent FUSE library and its development files for the "virtual mirror"
   filesystem

Build and installation instructions:

CMake is used internally for build control. It can be run in the source
directory and provides the usual configuration options. However, the
recommended way are out-of-source builds in a subdirectory like build/ that can
be removed to restart from scratch.  There are two convenience scripts that
automate this, build.sh and distclean.sh.
Workflow example:
 - run ./build.sh to configure and build (extra options are passed as cmake
   arguments, build results go to the builddir/ directory)
 - run "make -C builddir install" (maybe with DESTDIR=/tmp/somewhere/else)

Troubleshooting and integrator notes:
 - on configuration problems or to change cached settings afterwards, remove the
   builddir directory or just builddir/CMakeCache.txt in easy cases
 - the build system makes use of pkg-config to locate required software
   components. For locally built packages, you might need to adjust its
   path settings; see manpage for details.
 - Link Time Optimization may be broken with some compilers. If a GCC "internal
   compiler error" appears, first try -DUSE_LTO=off. Some installations also
   have a broken GNU linker, disabling LTO or installing gold (and restarting
   build process) might be advisable.
 - -DCMAKE_INSTALL_PREFIX=... specificies main directory to install (/usr/local is default)
 - -DCFGDIR=... (config directory, default is "etc" under CMAKE_INSTALL_PREFIX
   or just /etc if CMAKE_INSTALL_PREFIX is /usr)
 - -DDOCDIR=... and -DLIBDIR=... specify target directories for docs resp.
   supplementary files
 - see CMakeLists.txt for more paths that can be customized
 - Install halibut tool to rebuild the documentation, if needed

Configure to run on system startup:

This part heavily depends on the used init system.
 - For systemd configuration, the build process creates a service file and
   tmpfiles conf file in builddir/systemd directory. Their contents match the
   detected configuration. They need to be installed manually in the relevant
   folders (see systemd.unit(5), systemd-tmpfiles(8) and tmpfiles.d(5)
   manpages). Then, a local system user called "apt-cacher-ng" and the
   "apt-cacher-ng" group (with the new user therein) needs to be created, and
   new service is enabled with "systemctl enable apt-cacher-ng".
 - For sysvinit, see latest Debian package for examples (unpack with ar/tar or
   just mc).

Configuration:

 - create a directory for cached files and for log files
 - set ownership and directory permissions in a way which allows 
   read/write/mkdir actions to the user account used to run apt-cacher-ng later
 - copy conf directory to /etc/apt-cacher-ng or similar location. 
 - adapt acng.conf and other *.conf files in that configuration directory. 
   See user manual (doc/apt-cacher-ng.pdf or doc/README) for instructions.
 - run apt-cacher-ng -c yourConfigurationDirectory
 - visit Command-and-Control web interface of apt-cacher-ng, link can be found
   among other instructions at http://yourHostName:portNumber/ , which might be
   http://localhost:3142/ with the default configuration
 - for apt clients, there is a config snipped in contrib/10-apt-cacher-ng.conf
   which might be installed into /etc/apt/apt.conf.d/.

Developer notes:

There are some macros and shortcuts:
 - scripts/Makefile.release has some test and release rules, one could also run
   "make -f scripts/Makefile.release gendbs" to update built-in mirror
   configuration files
 - DEBUG argument of build.sh adds more verbosity and implicitly sets compiler
   defaults to produce debuggable binaries
 - -DDEBUG (set by above) to enable verbose debug logging. BE WARNED: this
   spams a lot and makes logs explode.
 - Certain environment variables influence cmake, like CXX (compiler to use),
   CXXFLAGS (compiler options, buid.sh won't set cmake defaults if this is
   set), LDFLAGS (compiler flags for linking)

LibTomCrypt notes:

 - LibTomCrypt does not have to be installed system-wide. It's sufficient to
   compile it as static library and set the environment variable TOMCRYPT_HOME
   pointing to LibTomCrypt's source directory
 - with the clang compiler, recent versions of LibTomCrypt might fail to build
   because of internal optimizations not understood by clang. The workaround
   for this is exporting the compiler definition -DLTC_NO_ROLC via CFLAGS