[Taler] Build process for wallets

[Christian Grothoff]

Hi!

Based on our telco last night, I revamped our build process(es) to make
it easier to build Taler wallets with minimal dependencies (i.e. no GNU
libextractor, no libltdl, no libz, etc.).

So here is how to use the new system:

1) To compile the Taler mint, proceed as before
    (the mint does require libltdl, and we don't care about minimizing
dependencies there so much)

2) To compile Taler wallets, you must now:
a) compile GNUnet with "--enable-taler-wallet" given to configure; this
will the NOT compile 99% of GNUnet and only
    generate a libgnunetutil_taler_wallet with about 1/3 of the code
from libgnunetutil

    Note that this library is NOT generated at all without this option.
Also note that you thus MUST compile GNUnet twice
    if you want to compile both mint and wallets.

b) compile taler-mint (!) with "--enable-wallet", this will then not
compile 95% of taler-wallet and only
    generate a libtalerutil_wallet that depends on
libgnunetutil_taler_wallet

c) compile your wallet and link it against libgnunetutil_taler_wallet


Right now, the result still depends on libunistring (via GNUnet's
'strings.c'), but that could be easily removed in the future if we
decide we don't need the respective routines.

Happy hacking!

Christian