Fix bogus handling of $(X) (i.e., ".exe") in isolationtester Makefile.

I'm not sure why commit 1eb1dde049 seems
to have made this start to fail on Cygwin when it never did before ---
but nonetheless, the coding was pretty bogus, and unlike the way we
handle $(X) anywhere else.  Per buildfarm.

src/test/isolation/Makefile

@@ -18,19 +18,19 @@ override LDLIBS := $(libpq_pgport) $(LDLIBS)
 
 OBJS =  specparse.o isolationtester.o
 
+all: isolationtester$(X) pg_isolation_regress$(X)
+
 submake-regress:
 	$(MAKE) -C $(top_builddir)/src/test/regress pg_regress.o
 
 pg_regress.o: | submake-regress
 	rm -f $@ && $(LN_S) $(top_builddir)/src/test/regress/pg_regress.o .
 
-pg_isolation_regress: isolation_main.o pg_regress.o
-	$(CC) $(CFLAGS) $^ $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)
-
-all: isolationtester pg_isolation_regress
+pg_isolation_regress$(X): isolation_main.o pg_regress.o
+	$(CC) $(CFLAGS) $^ $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@
 
-isolationtester: $(OBJS) | submake-libpq submake-libpgport
-	$(CC) $(CFLAGS) $(OBJS) $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)
+isolationtester$(X): $(OBJS) | submake-libpq submake-libpgport
+	$(CC) $(CFLAGS) $(OBJS) $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@
 
 distprep: specparse.c specscanner.c
 
@@ -41,8 +41,6 @@ distprep: specparse.c specscanner.c
 # important, otherwise make will choose the built-in rule for
 # gram.y=>gram.c.
 
-all: isolationtester$(X) pg_isolation_regress$(X)
-
 specparse.h: specparse.c ;
 
 # specscanner is compiled as part of specparse