bzip2: Fix test

bzip2 --version attempts to compress stdin.
gcc: Create a link from lib to lib64 for Linuxbrew

Otherwise /lib64 and /usr/lib64 will occur first in the search path.
Linuxbrew: Ignore .so. in check_non_libraries

Closes #121
Linuxbrew: Ignore brew doctor warnings for Mac OS

Closes #75
Linuxbrew: Factor out OS::GITHUB_REPOSITORY

Update cmd/doctor.rb and cmd/update.rb
Linuxbrew: Fix suspicious remote URL warning
Linuxbrew: Fix suspicious remote URL warning again
leveldb: Install shared library for Linuxbrew

Closes #120
curl: Disable LDAP for Linuxbrew

Closes #127
gcc: Create a GCC specs file for Linuxbrew

Set the library search path and the dynamic linker.