v.buffer: compile only if GEOS present

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@59986 15284696-431f-4ddb-bdfa-cd5b030d7da7

vector/v.buffer/Makefile

@@ -9,5 +9,6 @@ EXTRA_CFLAGS = $(VECT_CFLAGS) $(GEOSCFLAGS)
 
 include $(MODULE_TOPDIR)/include/Make/Module.make
 
+ifneq ($(strip $(USE_GEOS)),)
 default: cmd
-
+endif