In the 3.4 branch, in docs/building.md the instructions state that:
To build MongoDB, you will need: * A modern C++ compiler. One of the following is required. * GCC 4.8.2 or newer * Clang 3.4 (or Apple XCode 5.1.1 Clang) or newer * Visual Studio 2013 Update 2 or newer
which is out of date since according to the SConscript:
compiler_minimum_string = "GCC 5.3.0"
is required.