Use CARGO_ENCODED_RUSTFLAGS when it is set

[ColinFinck]

@cuviper Can this be merged and released?

[frankplus]

I would also like this to be merged and released. Thanks :)

[cuviper]

bors r+

[bors[bot]]

Build succeeded:

• Format
• Missing Target
• No Std
• Test (1.0.0)
• Test (1.10.0)
• Test (1.15.0)
• Test (1.20.0)
• Test (1.25.0)
• Test (1.30.0)
• Test (1.35.0)
• Test (1.40.0)
• Test (1.45.0)
• Test (1.5.0)
• Test (1.50.0)
• Test (1.55.0)
• Test (beta)
• Test (nightly)
• Test (stable)

[cuviper]

Published as 1.1.0, as well as a re-export as 0.1.8.