The libsodium source code is now embedded in the Serval DNA sourcecode
as a git subtree, so configure.ac and Makefile.in now recurse into it as
part of the Serval DNA build.
This simplifies the build instructions and eliminates all external
dependencies on libsodium development files and operating system
packages, returning Serval DNA to a self-contained component whose only
external dependencies are standard system libraries.
Remove the build-libsodium.sh script, as it is no longer needed.
swiftc(1) requires every -D option to be prefixed with -Xcc, which
is not easy to achieve using autoconf.
Now instead of setting configuration macros like HAVE_LSEEK64 on the
command line using -D, they are defined in config.h, which is generated
by ./configure, ignored by Git, and included by all headers and source
files that use any configuration macro.
The dependency on Makefile has been replaced with a dependency on
Makefile.in and config.h, which helps avoid some redundant full
recompiles after running ./configure, because the configure script
does not overwrit config.h if its content does not change.
