fix incorrect libstdc++ from GCC >= 5.1 check

Assume that libstdc++ from GCC >= 5.1 is being used only when compiling with GCC >= 5.1.

This unbreaks GCC 4.8.5 and 4.9.3 builds after https://codereview.chromium.org/1837563002/
landed.

Review URL: https://codereview.chromium.org/1863523005

Cr-Commit-Position: refs/heads/master@{#385416}
1 file changed