use unexport instead of export to clear perforce variables

SVN-Revision: 15708

include/toplevel.mk

@@ -27,10 +27,7 @@ export LD_LIBRARY_PATH:=$(if $(LD_LIBRARY_PATH),$(LD_LIBRARY_PATH):)$(STAGING_DI
 export DYLD_LIBRARY_PATH:=$(if $(DYLD_LIBRARY_PATH),$(DYLD_LIBRARY_PATH):)$(STAGING_DIR_HOST)/lib
 
 # prevent perforce from messing with the patch utility
-export P4PORT=
-export P4USER=
-export P4CONFIG=
-export P4CLIENT=
+unexport P4PORT P4USER P4CONFIG P4CLIENT
 
 # make sure that a predefined CFLAGS variable does not disturb packages
 export CFLAGS=