[PTFM-13637] Don't append -lgcc_eh when CC=clang

Summary: * When building jq with clang, omit the -lgcc_eh flag to avoid an error from the compiler.

Test Plan: Performed test builds on an OS X 10.7 builder with gcc and clang, and on Ubuntu 14.04 with gcc.

Reviewers: psung

Reviewed By: psung

Subscribers: akislyuk

Differential Revision: https://phabricator.dnanexus.com/D164