package/Makefile.in: use := for INSTALL

So it doesn't get reevaluated every time and for consistency with FLEX/BISON.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/Makefile.in

@@ -181,7 +181,7 @@ ifeq ($(BR2_STRIP_none),y)
 TARGET_STRIP=true -Not_stripping
 STRIPCMD=$(TARGET_STRIP)
 endif
-INSTALL=$(shell which install || type -p install)
+INSTALL:=$(shell which install || type -p install)
 FLEX:=$(shell which flex || type -p flex)
 BISON:=$(shell which bison || type -p bison)