Fix make install when all of ffmpeg/ffplay/ffserver are disabled

Originally committed as revision 20501 to svn://svn.ffmpeg.org/ffmpeg/trunk

Makefile

@@ -8,7 +8,7 @@ PROGS-$(CONFIG_FFMPEG)   += ffmpeg
 PROGS-$(CONFIG_FFPLAY)   += ffplay
 PROGS-$(CONFIG_FFSERVER) += ffserver
 
-PROGS       = $(addsuffix   $(EXESUF), $(PROGS-yes))
+PROGS      := $(addsuffix   $(EXESUF), $(PROGS-yes))
 PROGS_G     = $(addsuffix _g$(EXESUF), $(PROGS-yes))
 OBJS        = $(addsuffix .o,          $(PROGS-yes)) cmdutils.o
 MANPAGES    = $(addprefix doc/, $(addsuffix .1, $(PROGS-yes)))