depends: add shared dependency builder

See the README's in depends for documentation

depends/README.usage

@@ -0,0 +1,29 @@
+To build dependencies for the current arch+OS:
+    make
+To build for another arch/OS:
+  make HOST=host-platform-triplet && make HOST=host-platform-triplet
+  (For example: make HOST=i686-w64-mingw32 -j4)
+
+A prefix will be generated that's suitable for plugging into Bitcoin's
+configure. In the above example, a dir named i686-w64-mingw32 will be
+created. To use it for Bitcoin:
+
+./configure --prefix=`pwd`/depends/i686-w64-mingw32
+
+No other options are needed, the paths are automatically configured.
+
+Dependency Options:
+The following can be set when running make: make FOO=bar
+
+SOURCES_PATH: downloaded sources will be placed here
+BASE_CACHE: built packages will be placed here
+SDK_PATH: Path where sdk's can be found (used by OSX)
+NO_QT: Don't download/build/cache qt and its dependencies
+NO_WALLET: Don't download/build/cache libs needed to enable the wallet
+NO_UPNP: Don't download/build/cache packages needed for enabling upnp
+
+If some packages are not built, for example 'make NO_WALLET=1', the appropriate
+options will be passed to bitcoin's configure. In this case, --disable-wallet.
+
+Additional targets:
+download: run 'make download' to fetch sources without building them