Fix build without glibc

Summary: The preprocessor does not follow normal rules of && evaluation, tries to evaluate __GLIBC_PREREQ(2, 12) even though the defined() check fails.  This breaks the build if __GLIBC_PREREQ is absent.

Test Plan: Try adding #undef __GLIBC_PREREQ above the offending line, build no longer breaks

Reviewed By: igor

Blame Rev: 4c81383628db46d35b674000a3668b5a9a2498a6