Linuxbrew: Set LD_RUN_PATH unless using glibc

When glibc is installed, both the dynamlic linker and rpath must be set.

Closes Homebrew/linuxbrew#150
Linuxbrew README.md: Do not set LD_LIBRARY_PATH

Closes Homebrew/linuxbrew#150
git-flow-avh: fix zsh zsh_completion.
git-flow-avh: Support Linux systems.
git: only show OS X keychain caveat on OSX systems.