Speed up parallel builds

- make should be $(MAKE)
 - add + in front of rules spawning long-lasted make process in a
subshell. (This would not be needed with $(MAKE) -C .. target, but our
makefiles do not handle that because they use $(PWD))
 - split the main 'all' rule as all 4 targets are independant
 - fix dependencies where appropriate for parallelism

Extra, not speed-related changes:
 - remove some double-colon for targets as they do not need it

This cuts build time from 5s to 1.5s on a laptop with -j4, and more
importantly from 85s to 35s on a KNL node.
As a bonus, the fixed dependencies removes the need to clean before
rebuilding all the time. Probably.

executer/kernel/mcoverlayfs/Makefile.in

@@ -33,7 +33,7 @@ endif
 
 modules:
 ifneq ($(BUILD_MODULE),none)
-	@(cd $(BUILD_MODULE); make modules)
+	+@(cd $(BUILD_MODULE); make modules)
 endif
 
 clean:

executer/user/Makefile.in

@@ -53,7 +53,7 @@ libsched_yield: libsched_yield.c
 	$(CC) -shared -fPIC -Wl,-soname,sched_yield.so.1 -o libsched_yield.so.1.0.0 $^ -lc -ldl
 
 libmcexec.a::
-	(cd arch/${ARCH}; make)
+	+(cd arch/${ARCH}; $(MAKE))
 
 libqlmpi.so: qlmpilib.c
 	$(MCC) $(CFLAGS) $(LDFLAGS) -shared -fPIC -o $@ $<
@@ -77,13 +77,13 @@ ql_talker: ql_talker.o
 	$(CC) $^ $(CFLAGS) -o $@
 
 clean::
-	(cd arch/${ARCH}; make clean)
+	(cd arch/${ARCH}; $(MAKE) clean)
 	$(RM) $(TARGET) *.o
 
 .PHONY: all clean install
 
 install::
-	(cd arch/${ARCH}; make install)
+	(cd arch/${ARCH}; $(MAKE) install)
 	mkdir -p -m 755 $(BINDIR)
 	install -m 755 mcexec $(BINDIR)
 	mkdir -p -m 755 $(MCKERNEL_LIBDIR)