Cross compiling with a makefile on Mac - ios

Question

I've been writing a makefile which will need to compile for multiple platforms. Right now I only need to compile for iOS, but as I already know that I'll need to support Mac OS X in a near future I've also been looking on how to compile for that as well. Notice that I'm not using Xcode. That's not an option as the project will also have to support other platforms.
I already have a makefile compiling for iPhone. It's working (tested on the simulator, even) but I still have a few doubts.
Specifically I've read about many tools and right now I'm a bit confused about some of them:
xcodebuild and clang I get clang, it's a compiler, but what about xcodebuild? Does it just use clang for compiling and also runs some tests? Are there upsides to using? Downsides?
libtool vs ar pretty much the same thing. How do these differ? I've read that ar should be followed by a call to ran lib (although it's not clear on the why), so there must be downsides, but which? Edit: forgot to include lipo in the discussion
Finally, and I hope I'm not getting off topic, I'm generating fat files for iphoneos and iphonesimulator. Here's how; first I compile for each platform:
ifeq ($(ARCH), iphoneos)
ARCHFLAGS = -arch armv7 -arch armv7s -arch arm64
else
ARCHFLAGS = -arch i386 -arch x86_64
endif
ifeq ($(ARCH), iphoneos)
VERFLAGS = -miphoneos-version-min=7.0
else
VERFLAGS = -mios-simulator-version-min=7.0
endif
CC = $(shell xcrun --sdk $(ARCH) --find clang)
CFLAGS := -isysroot $(shell xcrun --sdk $(ARCH) --show-sdk-path)
CFLAGS += $(ARCHFLAGS)
$(OBJDIR)/%.o: $(SRCDIR)/%.cpp
$(CC) $(CFLAGS) -c $< -o $#
Some flags, variables, and rules are omitted. This generates two sets of .o files. I then use ar (should it be libtool?):
AR = ar
ARFLAGS = crv
$(BINARY): $(OBJECTS) # This rule depends on the previous one
$(AR) $(ARFLAGS) $(BINARY) $(OBJECTS)
By now I have a static library (.a) file for each platform. I proceed to generate the fat file:
$(LIBTOOL) $(LIBTOOLFLAGS) -o $(call libfile,iphone,debug) $(call libfile,iphoneos,debug) $(call libfile,iphonesimulator,debug)
Note: libfile is a function which returns the file name for the given configuration.
Two questions now:
Could I just skip the generation of the two separate .a files and jump to the the fat file by specifying ARCHFLAGS = -arch armv7 -arch armv7s -arch arm64 -arch i386 -arch x86_64? That is, all platforms in a single call. Is it the same thing?
This was the closest I got to listing supported architectures (not a good option, I'd say). Could I just list all (or a subset) of them, generate an "obese" (lol) file and expect the compiler to optimize the file's size when linked against an actual implementation? Or are fat files shipped with the final product?
Thank you for reading so far.
Best.