qmake|Unix: Added deng_nosnowberry for excluding Snowberry

The deng_nosnowberry CONFIG option will prevent a generic
Unix build from installing the Snowberry files.

This only affects generic Unix because on other platforms the
installation of Snowberry is handled by the platform_release.py
script. (Similarly, "deng_snowberry" only affects generic Unix.)

(based on patch by vvv1)

doomsday/config.pri

@@ -20,6 +20,7 @@
 # - deng_fmod               Build the FMOD Ex sound driver
 # - deng_nofixedasm         Disable assembler fixed-point math
 # - deng_nosdlmixer         Disable SDL_mixer; use dummy driver as default
+# - deng_nosnowberry        (Unix) Exclude Snowberry from installation
 # - deng_openal             Build the OpenAL sound driver
 # - deng_packres            Package the Doomsday resources
 # - deng_rangecheck         Parameter range checking/value assertions
@@ -112,7 +113,10 @@ unix {
 }
 unix:!macx {
     # Generic Unix build options.
-    CONFIG += deng_nofixedasm deng_snowberry deng_packres
+    CONFIG += deng_nofixedasm deng_packres
+    !deng_nosnowberry {
+        CONFIG += deng_snowberry
+    }
 
     exists(/etc/apt) {
         # Choose the apt repository to include in the distribution.