xcrun: actually try fallbacks

xcrun has a lot of fallbacks if the first case fails but never actually
reaches them on CLT systems since it doesn't check the validity of the
first path before executing it.

(When should it reach these? Mainly for non-Xcode compilers we
support, e.g. apple-gcc42 which can be found by xcrun but which isn't
in /usr/bin)

The xcrun invocation also needed chomping.
superenv: --use-gcc should specify gcc-4.2

Since 'gcc' is a symlink to 'llvm-gcc' on Xcode 4.3+, --use-gcc and
--use-llvm were doing exactly the same thing. Combined with the
previous commit, this allows users with either a leftover
/usr/bin/gcc-4.2 or a homebrewed apple-gcc42 to build with gcc.

This doesn't however fix Xcode-only systems with apple-gcc42.

Fixes #17243.
ffmpeg: use new optional deps

Keeps a few of the old option descriptions where they provided extra
information, but many of them were not really useful or accurate.
libsvm 3.16

 - Use archive URL

Closes #17340.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
MeCab 0.995

Closes #17335.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
youtube-dl 2013.01.28

Closes #17342.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
doctor: fix transposed git command

Closes #17338.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
gtk+: atk is not optional

Fixes #17346.
Update require
audit: hint about depends_on Class
boost: style nits
boost149: style nits
cmake: style nits
coq: style nits
cvs2svn: style nits
drizzle: style nits
elixir: style nits
ghc: style nits
google-js-test: style nits
hyperestraier: style nits