Makefile: Don't rebuild targets multiple times on parallel build.

The nets Makefile previously called the "all" target in the net_imap
sub-Makefile, which in turn would build all the files in that
subdirectory. However, this resulted in duplicate builds when
parallelizing a build (e.g. make -j2), since the net_imap/%.o
target would get called multiple times (depending on -j number),
which effectively called $(SUBMAKE) -C net_imap all multiple times.
Instead of doing this, call the specific target name in the
sub-Makefile instead, so that the nets Makefile only rebuilds an
individual target.

nets/Makefile

@@ -32,10 +32,11 @@ IMAP_OBJ = $(patsubst %.c,%.o,$(IMAP_SRC))
 
 # XXX if nets/net_imap/imap.h is modified, only net_imap.c is automatically recompiled,
 # not any nets/net_imap/*.c source files that include it
+# XXX If using -j[>1], some files in net_imap/ are recompiled multiple times (parallel build bug)
 
 # Subdirectory with components for net_imap
 net_imap/%.o: $(IMAP_SRC)
-	@+$(SUBMAKE) --no-builtin-rules -C net_imap all
+	@+$(SUBMAKE) --no-builtin-rules -C net_imap $(basename $(notdir $@)).o
 
 net_imap.so : net_imap.o $(IMAP_OBJ)
 	@echo "  [LD] $< $(IMAP_OBJ) -> $@"

nets/net_imap/Makefile

@@ -6,8 +6,6 @@ DEPENDS := $(patsubst %.c,%.d,$(IMAP_SRC))
 # the include directory is in the parent's parent
 INC = -I../..
 
-all: $(MOD_SO)
-
 -include $(DEPENDS)
 
 $(DEPENDS):