Setting C++ compile flags in xcode
Instead of putting these under "Other C/C++ Flags", they should go under "Other Linker Flags" (in the Linking section).
(Note that my XCode is old, so it may be slightly different for your version.)
You might wonder, why is this necessary?
Well, when you build your project, there are several stages to go through. The most basic breakdown is into compiling and linking. (They could perhaps be broken down further, but that's the important distinction here.)
The compiler takes a source file (eg, example.cpp) and outputs an object file (such as example.o). An object file is not executable. When compiling, the compiler generally only knows about the one source file that it's currently processing. Thus the compiler doesn't need to know which libraries you're using - all it needs to know is where the header files are.
The linker takes one or more object files and combines them together to create an executable binary. At this point, it must also resolve any external symbols not defined in your code - for example, symbols defined in an external library. For that reason, the linker needs to know about any libraries you're using.
The compiler does not know what to do with an -l or -L flag - they're not relevant to the process of compiling your code into an object file.
When you invoke gcc from the command-line like you demonstrated, it automatically invokes the linker for you and forwards those -l and -L flags to it. Because of this, no object file is produced on disk, and you get an executable file.
However, when you build through XCode, it does things a little differently. It invokes the compiler once for each of your source files, producing an object file like I described above. (This is the reason why you can specify extra compiler flags for specific source files in the Build Phases -> Compile Sources section.) Because the compiler has been asked to produce an object file, it does not invoke the linker, and since you're trying to pass it flags that should be forwarded to the linker, you get that warning that the flags are not used.
Once all the source files have successfully compiled, XCode next invokes the linker directly to combine them all into a single executable binary. This is the stage that needs to know about your libraries. (Incidentally, in any large project, this method is generally preferable even if you're not using XCode.)