Bump useable clang version to 318

Build 211 can build most things, but I've seen intermittent
miscompilation and slower code. Also it comes from Xcode 4.2, so most
people don't have it anyway. Let's use 218 as the minimum viable
version.
CompilerSelector: don't use non-existent compilers
Isolate compiler selector tests
Add new tests for fails_with DSL
Fix chicken scheme imports
Simplify setting this ivar
Split out and fix LanguageModuleDependency tests
Don't use "assert_not_nil"
Shrink requirements.rb
Fix requires in test_x11_dependency.rb
Add test for differing module and import name
Suppress warnings in test_pathname_install
Fix missing require in x11_dependency.rb

Fixes #18918.
weechat: disable gettext/NLS for now

This is a workaround for #18722.
tap/untap: use Array#length, not #count

Array#count was apparently introduced in 1.8.7.
pngcrush 1.7.56

Closes #18930.
Closes #18928.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
webp 0.3.0

Closes #18927.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
jenkins 1.509
ncdc 1.16.1

Closes #18933.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Document `brew deps --installed`