Remove ECDSA and repair Clang build

[twopir]

This is literally just git cherry-pick dcb1dee5c47..32c05f3c from zooko's 85_disable_asm branch at https://github.com/zooko/pycryptopp/tree/85_disable_asm — these two changes are kinda intertwined.

[daira]

Reviewing.

[daira]

Finished reviewing. Overall -1 for now, see the comments on [85fbeba] and [2f11e66], and split out the ECDSA removal into a separate pull request.

[warner]

I successfully tested just the first two commits (git checkout cd7a209) on a debian/sid box in two modes:

• python setup.py build (embedded cryptopp)
• apt-get install libcrypto++-dev, python setup.py build --disable-embedded-cryptopp

and also the same commit with just the embedded cryptopp on OS-X 10.10.5 . All three configs worked.

How about we land just the first two commits and move the rest to a different PR?

[zooko]

I'm good with landing the first two commits and moving the rest to a different PR, but I'd really prefer it if I could see a Mac OS 10.10.5 buildslave go from red to green when we land them (because https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2527).

[warner]

Buildslave is online (https://tahoe-lafs.org/buildbot-pycryptopp/waterfall) and red, as expected. Land away.

[zooko]

@warner Can you reconnect your Mac OS X buildslave, please?

[zooko]

#27 landed the first two patches.

[warner]

I'll try to spin it back up on wednesday night. It's running 10.11 now, though.

[warner]

BTW, buildslave is back online as of yesterday (https://tahoe-lafs.org/buildbot-pycryptopp/waterfall), OS-X (10.11) builder is currently green