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)
FALLBACK_DOWNLOAD_PATH: If a source file can't be fetched, try here before giving up
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
DEBUG: disable some optimizations and enable more runtime checking

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 all sources without building them
download-osx: run 'make download-osx' to fetch all sources needed for osx builds
download-win: run 'make download-win' to fetch all sources needed for win builds
download-linux: run 'make download-linux' to fetch all sources needed for linux builds
