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.

Makefile.in

@@ -4,13 +4,21 @@ INCDIR =  @INCDIR@
 ETCDIR = @ETCDIR@
 MANDIR = @MANDIR@
 
-all::
-	@(cd executer/kernel/mcctrl; make modules)
-	@(cd executer/kernel/mcoverlayfs; make modules)
-	@(cd executer/user; make)
-	@case "$(TARGET)" in \
+all: executer-mcctrl executer-mcoverlayfs executer-user mckernel
+
+executer-mcctrl:
+	+@(cd executer/kernel/mcctrl; $(MAKE) modules)
+
+executer-mcoverlayfs:
+	+@(cd executer/kernel/mcoverlayfs; $(MAKE) modules)
+
+executer-user:
+	+@(cd executer/user; $(MAKE))
+
+mckernel:
+	+@case "$(TARGET)" in \
 	    attached-mic | builtin-x86 | builtin-mic | smp-x86 | smp-arm64) \
-		(cd kernel; make) \
+		(cd kernel; $(MAKE)) \
 		;; \
 	    *) \
 		echo "unknown target $(TARGET)" >&2 \
@@ -18,13 +26,13 @@ all::
 		;; \
 	esac
 
-install::
-	@(cd executer/kernel/mcctrl; make install)
-	@(cd executer/kernel/mcoverlayfs; make install)
-	@(cd executer/user; make install)
+install:
+	@(cd executer/kernel/mcctrl; $(MAKE) install)
+	@(cd executer/kernel/mcoverlayfs; $(MAKE) install)
+	@(cd executer/user; $(MAKE) install)
 	@case "$(TARGET)" in \
 	    attached-mic | builtin-x86 | builtin-mic | smp-x86 | smp-arm64) \
-		(cd kernel; make install) \
+		(cd kernel; $(MAKE) install) \
 		;; \
 	    *) \
 		echo "unknown target $(TARGET)" >&2 \
@@ -67,13 +75,13 @@ install::
 		;; \
 	esac
 
-clean::
-	@(cd executer/kernel/mcctrl; make clean)
-	@(cd executer/kernel/mcoverlayfs; make clean)
-	@(cd executer/user; make clean)
+clean:
+	@(cd executer/kernel/mcctrl; $(MAKE) clean)
+	@(cd executer/kernel/mcoverlayfs; $(MAKE) clean)
+	@(cd executer/user; $(MAKE) clean)
 	@case "$(TARGET)" in \
 	    attached-mic | builtin-x86 | builtin-mic | smp-x86 | smp-arm64) \
-		(cd kernel; make clean) \
+		(cd kernel; $(MAKE) clean) \
 		;; \
 	    *) \
 		echo "unknown target $(TARGET)" >&2 \