refactor the OS X build system to use latest libjpeg/libpng libraries

Earlier, the bundled versions of the above libraries were used, which is
not safe as there might be security vulnerabilities in those versions.
The framework introduced in 9db4dce10d8ba81cb854d6994a842b6c9d149726 to
support building dependent libraries for Windows was reused to download
and compile the latest versions of the above libraries.

In addition, the build system was refactored to use "make install" support
which was introduced in bd16273f806759d82c9f64abaa2c093a76442dad for Linux.
generate native PKG for OS X using "fpm"

On OS X, packages are very limited and do not have the functionality that is
provided by package managers on Linux/BSD systems -- installation is supported,
but no uninstallation. Also, the compression support for packages is very poor
-- a 92MiB package is generated instead of a 14MiB tarball compressed using xz.

Thus, we use a very ugly hack to get the package down to a reasonable size. We
actually create a compressed tarball using xz (as before) and include a after-
install script which decompresses this tarball into /usr/local. To support the
scenario where the end-user might not have xz installed on the machine, a copy
of unxz is also bundled in the package.
update the fpm script in the schroot environments