build: add an option for enabling glibc back-compat

Using "./configure --enable-glibc-back-compat" will attempt to be
compatible with a target running glibc prior to 2.14.
Do not merge. build: add test-case for libc-compat

This test intentionally causes a buffer overflow, and passes if it is caught
and aborted as expected.

Unfortunately, there doesn't seem to be a reasonble way to silence glibc's
backtrace output, so the test appears to crash when it's actually passing.

Any Ideas?