As Ansi C is very lightweight, I recommend the latest version of LoadRunner, version 12. A 50 user non expiring version is available and feature-wise it buries tools like JMETER and Gatling. You could also check out SOASTA, a 100 thread non-expiring version is available at no charge. Also, feature wise, this is a commercial tool built for testing. SOASTA is a bit heavier on the per virtual user front as it uses JavaScript as it's core language.
Both tools have a long track record of success and defined capabilities in the CI space with Jenkins and similar.
If you're at the point where the financial risk is so large with failure that performance testing is now mandatory then what you will find when you measure the financial risk and compare it to the cost of any market tool that the tool cost is typically the size of a boil on the butt of a mosquito when compared to a risk the size of the cow the mosquito is feeding on.