I'm switching from nasm to fasm, because the latter has a more expressive macro facility. The downside of fasm is that it doesn't support OSX. But, with some Google-fu and Makefile-fu, I have made it support OSX, with the same level of portability and convenience that I had previously achieved with nasm! (The key insights were found in this forum thread.)

In order to use the Makefile here, you will need objconv, and you will also need an already-working fasm. If you're on OSX, that might be quite difficult, so I'm including a working OSX fasm binary as a download in this repository.

Verify that you have these installed:

$ fasm
flat assembler  version 1.71.17
usage: fasm <source> [output]
$ objconv

Object file converter version 2.32 for x86 and x86-64 platforms.
Copyright (c) 2013 by Agner Fog. Gnu General Public License.

Then, it's as simple as

$ git clone https://github.com/davidad/fasm_example_64.git
Cloning into 'fasm_example_64'...
remote: Counting objects: 13, done.
remote: Compressing objects: 100% (10/10), done.
remote: Total 13 (delta 3), reused 13 (delta 3)
Unpacking objects: 100% (13/13), done.
$ cd fasm_example_64
$ make
ln -s osx.inc syscalls.inc
fasm hello64.asm hello64.o
flat assembler  version 1.71.17  (16384 kilobytes memory)
2 passes, 467 bytes.
objconv -fmacho -ar:start:_start -nu hello64.o hello64.mach.o

Input file: hello64.o, output file: hello64.mach.o
Converting from ELF64 to Mach-O Little Endian64
Adding leading underscores to symbol names

  0 Debug sections removed
  0 Exception sections removed
  0 Changes in leading underscores on symbol names
  1 Symbol names changed
  1 Public symbol names aliased
ld hello64.mach.o -o hello64.mach.exe
ld: warning: -macosx_version_min not specified, assuming 10.6
ln -f hello64.mach.exe hello64
rm hello64.o hello64.mach.exe hello64.mach.o
$ ./hello64
Hello world!