Allow custom command for compiling user source code

[mikebentley15]

Currently it is supported to add user source files under test to be compiled with the tests. However, not all build systems are simple enough to do that easily (e.g autogenerated source files, or large amounts of static or shared libraries).

What if instead we allow the user to compile their source code using their own build systems for things like autogenerated source files and generating different compilations of shared and static libraries. This user-provided custom command would need to be given variables set to useful values such as the compiler, optimization level, and compiler switches. The user-provided command would also need to ensure that their custom command is reentrant (i.e. multiple invocations simultaneously are safe when done with different tuples of (compiler, optimization level, switches)), most likely by specifying a unique directory or set of filenames for generated files.

This issue would be hard to resolve before implementing a recursive Makefile (see issue #72).

[mikebentley15]

Issue #72 has already been resolved, so this issue is easier to achieve.