Fix gmake 4.4 "pattern recipe did not update peer target" warnings

Due to an upcoming change in how GNU make will handle pattern
rules with multiple targets, GNU make 4.4 warns when pattern
recipies don't update all targets listed.  See
https://lists.gnu.org/archive/html/help-make/2022-10/msg00020.html
for more information.

Such a case existed in bcftools' Makefile in a rule intended to
make either .dll or .cygdll targets depending on the value of
the PLUGIN_EXT variable.  As PLUGIN_EXT could also be .so (handled
by another rule) the easiest solution is to spilt the single rule
into two, each having a single pattern target.

Makefile

@@ -196,7 +196,10 @@ libbcftools.a: $(OBJS)
 
 vcfplugin.o: EXTRA_CPPFLAGS += -DPLUGINPATH='"$(pluginpath)"'
 
-%.dll %.cygdll: %.c version.h version.c libbcftools.a $(HTSLIB_DLL)
+%.dll: %.c version.h version.c libbcftools.a $(HTSLIB_DLL)
+	$(CC) $(PLUGIN_FLAGS) $(CFLAGS) $(ALL_CPPFLAGS) $(EXTRA_CPPFLAGS) $(LDFLAGS) -o $@ version.c $< $(PLUGIN_LIBS)
+
+%.cygdll: %.c version.h version.c libbcftools.a $(HTSLIB_DLL)
 	$(CC) $(PLUGIN_FLAGS) $(CFLAGS) $(ALL_CPPFLAGS) $(EXTRA_CPPFLAGS) $(LDFLAGS) -o $@ version.c $< $(PLUGIN_LIBS)
 
 %.so: %.c version.h version.c