add c++11 to performance tests (#92)
add c++11 to performance tests (#92)

Builds in environments where the C++11 standard is not enabled by default
encounter problems building the performance tests in the `bench` target.
This change adds C++11 to the compilation flags for performance tests so
that they can be built without issue.

This fixes #91, which can be referenced for more details.

Co-authored-by: Aravind Padmanabhan <98fce83ca112b2e5fa925457ab41cd3b46a083c1@netcore.co.in>
goatshriek/stumpless,goatshriek/stumpless,goatshriek/stumpless,goatshriek/stumpless