Add addBuildOpt helper function for building option string

addBuildOpt should be called with the build option which is NOT
surrounded by spaces since it will add spaces to separate options
itself, if necessary.
Replace 'strcat (buildOptStr,...' with 'addBuildOpt(buildOptStr,...'

This fixes the invalid program options kernel build error on OSX
10.9.3 (cf. issue #37).
addBuildOpt: use strncat instead of strlcat

Linux does not have strlcat.
Fix c&p bug in rotmg gen introduced by addBuildOpt change
Merge pull request #38 from gicmo/addBuildOpt

Add small helper function to build up kernel build options
:+1: