Ask HN: Lean and mean back-end programming

If I were writing a Windows app, the leanest language (barring x86 assembly) would be C written to the Win32 API. Analogously, what would be the leanest way to program on the server side. We're building a web-based application with some server side components and would like to program those in C++. What would be the best choice for platform libraries and tools. Our objective is to produce the leanest code so that we can service lots of clients with minimal compute power.