OWGUIBUILD build variable

Add condition code to enable/disable compiling GUI/IDE/Browser
components and browse headers. Default on in the sample setvars.sh in
consideration of others who are used to compiling these components.
Disabling it saves some compile time and removes the need to bring in
DOSBox or DOSemu. The remainder of the compiler (non-GUI command line
tools) still compile.

(how's this?)