HarfBuzz integration should be functional now, so to give it more wider
testing it is made now a required dependency on non-Windows non-Mac OSs.
By default text layout is now done by HarfBuzz but ICU LayoutEngine is
kept as a fallback and can be enabled with SAL_USE_ICULE env variable.

After 4.1.x is branched, ICU LayoutEngine should be removed completely.

Change-Id: I4fe3beeaf6092f33dd436906c11b83aeafdfbd5d