Add filename and SHA256 hash for Windows Rust package
When running `make download` in the `depends` directory, the `download-win` target (which `download` depends on) generates an error when it runs the commands defined in `check_or_remove_sources`:
```Makefile
sha256sum: /home/vagrant/zcash/depends/work/download/rust-1.16.0/..hash: no properly formatted SHA256 checksum lines found
funcs.mk:242: recipe for target '/home/vagrant/gitian-builder/cache/common/download-stamps/.stamp_fetched-rust-.hash' failed
make[1]: *** [/home/vagrant/gitian-builder/cache/common/download-stamps/.stamp_fetched-rust-.hash] Error 1
make[1]: Leaving directory '/home/vagrant/zcash/depends'
Makefile:153: recipe for target 'download-win' failed
make: *** [download-win] Error 2
```
The reason for the error is that `depends/packages/rust.mk` defines `rust_file_name_linux` and `rust_file_name_darwin` but leaves `rust_file_name_mingw32` undefined.
A directory of available rust downloads is here: https://static.rust-lang.org/dist/index.html
The closest windows analog in that list (using the same version number as currently defined in `rust.mk`) appears to be `rust-mingw-1.16.0-x86_64-pc-windows-gnu.tar.gz`. A corresponding sha256 value is also given in `rust-mingw-1.16.0-x86_64-pc-windows-gnu.tar.gz.sha256`.
After adding these values to `rust.mk`, the rust-mingw tar package was downloaded along with the rest of the dependencies and the above error message went away.
PATH variable containing spaces cause build failure
Spaces in PATH variable is creating build issues (observed on macOS). For example "VMware Fusion" adds itself to PATH like `/Applications/VMware Fusion.app/Contents/Public`.
Currently Travis's wget fails fetching qrencode:
Fetching qrencode...
ERROR: no certificate subject alternative name matches
requested host name `fukuchi.org'.
To connect to fukuchi.org insecurely, use `--no-check-certificate'.
OpenSSL: error:14077438:SSL routines:SSL23_GET_SERVER_HELLO:tlsv1 alert internal error
Unable to establish SSL connection.
make: *** [/home/travis/build/luke-jr/bitcoin/depends/sources/download-stamps/.stamp_fetched-qrencode-qrencode-3.4.4.tar.bz2.hash] Error 4
Closes#2279. Configures CMake to enable C++11, build static libaries
and only build cpp bindings with minimal dependencies. Documentation,
examples, tests and other language bindings are no longer built.
CMake will no longer try to find commands and packages which are not
required for building the target.
Debian 8 stable ships with gcc 4.9.2 and cmake 3.0.2. Previously
the depends package used CMAKE_CXX_STANDARD to tell cmake to use
C++11, but the option requires cmakes 3.1+. To resolve the issue
we now update relevant CMakeLists.txt and set CMAKE_CXX_FLAGS.
