build, docs: Improve macOS building

- New docker command for quickly running tasks in a Linux environment.
- Updated docs with macOS specific details.
- Fixed some test issues.
- Add (fallible) macOS test target for Travis.

test/test-examples.sh

@@ -19,7 +19,7 @@ make build
 buildout='test-examples.out'
 # make symlink to outside of package
 linkto="`pwd`/examples/lib/"
-tmpdir="`mktemp --tmpdir -d tmp.XXX`"	# get a dir outside of the main package
+tmpdir="`$mktemp --tmpdir -d tmp.XXX`"	# get a dir outside of the main package
 cd "$tmpdir"
 ln -s "$linkto"	# symlink outside of dir
 cd `basename "$linkto"`