glibc: Fix the test which could incorrectly pass

Look for the string 'Usage' rather than 'ld.so, which passed this case:
Inconsistency detected by ld.so: get-dynamic-info.h: 134: elf_get_dynamic_info:
Assertion `info[15] == ((void *)0)' failed!
Linuxbrew: Do not set rpath for glibc

Closes #126
Linuxbrew: keg_only :provided_by_osx only on Mac

keg_only :provided_by_osx is invalid on other systems.
Merge branch 'master' of https://github.com/Homebrew/linuxbrew