Build fls.o only when AC_REPLACE_FUNCS so dictates via $(LIBOBJS).

By building it unconditionally, libpgport inadvertently replaced any
libc version of the function.  This is essentially a code cleanup; any
effect on performance is almost surely too small to notice.

src/port/Makefile

@@ -30,7 +30,7 @@ include $(top_builddir)/src/Makefile.global
 override CPPFLAGS := -I$(top_builddir)/src/port -DFRONTEND $(CPPFLAGS)
 LIBS += $(PTHREAD_LIBS)
 
-OBJS = $(LIBOBJS) chklocale.o dirmod.o erand48.o fls.o inet_net_ntop.o \
+OBJS = $(LIBOBJS) chklocale.o dirmod.o erand48.o inet_net_ntop.o \
 	noblock.o path.o pgcheckdir.o pgmkdirp.o pgsleep.o \
 	pgstrcasecmp.o pqsignal.o \
 	qsort.o qsort_arg.o quotes.o sprompt.o tar.o thread.o