Makefile -- Platform Dependency
Conner, I have a few minor points.  


Firstly, you have no includes (ie, no -Idir). Do you really have no headers? 
You should make the non-targets 'PHONY' so make knows they will never exist:

.PHONY clean
clean: 
        rm -f ...


Same with all, server and client
It is not normal to clean everything on each build - usually you let make decide what is out of date and needs to be rebuilt.  For that you need to have all the dependencies declared to make (eg headers)
client and server depend upon the build directories and the executables, but not directly upon the sources.
the clean target remakes the build directories.  To me, these directories should be made when you build the targets:

$(CLIENT_OBJ_DIR) $(SERVER_OBJ_DIR):
        mkdir -p $@ 

client: $(CLIENT_OBJ_DIR) $(CLIENT_EXECUTABLE)

server: $(SERVER_OBJ_DIR) $(SERVER_EXECUTABLE)

-Wall does not really give you 'all' warnings. There are many more that are worth enabling.