build-userland: multithreading

Looking ahead for when I'll move cpp-cheat C++ slowness in.

First tried with concurrent as in:
https://stackoverflow.com/questions/19369724/the-right-way-to-limit-maximum-number-of-threads-running-at-once/19370282#19370282
Implementation was easy, but I can't find out how to exit immediately on error,
so I came up with this setup instead.

userland/Makefile

@@ -1,75 +0,0 @@
-.PHONY: all clean mkdir
-
-ARCH = $(shell uname -m)
-CCFLAGS = -ggdb3 -I$(COMMON_DIR) -O0 -Wall -Werror -Wextra -Wno-unused-function $(CCFLAGS_EXTRA) $(CCFLAGS_SCRIPT)
-CFLAGS = -fopenmp -std=c99 $(CCFLAGS) $(CFLAGS_EXTRA)
-CXXFLAGS = -std=c++17 $(CCFLAGS) $(CXXFLAGS_EXTRA)
-# -Wno-unused-function for function definitions on headers,
-#  because we are lazy to make a shared object. TODO.
-COMMON_DIR = $(CURDIR)/..
-COMMON_BASENAME = lkmc
-COMMON_OBJ = $(OUT_DIR)/$(COMMON_BASENAME)$(OBJ_EXT)
-IN_EXT_ASM = .S
-IN_EXT_C = .c
-IN_EXT_CXX = .cpp
-LIBS = -lm
-OBJ_EXT = .o
-OUT_EXT = .out
-OUT_DIR = $(CURDIR)
-
--include params.mk
-
-OUTS := $(foreach IN_EXT,$(IN_EXT_ASM) $(IN_EXT_C) $(IN_EXT_CXX),$(addsuffix $(OUT_EXT), $(basename $(wildcard *$(IN_EXT)))))
-ifeq ($(HAS_EIGEN),y)
-	CXXFLAGS_EXTRA += -I$(STAGING_DIR)/usr/include/eigen3
-	# TODO: was failing with:
-	# fatal error: Eigen/Dense: No such file or directory as of
-	# 975ce0723ee3fa1fea1766e6683e2f3acb8558d6
-	# http://lists.busybox.net/pipermail/buildroot/2018-June/222914.html
-	#CXXFLAGS_EXTRA += $(shell $(PKG_CONFIG) --cflags eigen3)
-else
-	OUTS := $(filter-out eigen_%$(OUT_EXT),$(OUTS))
-endif
-ifeq ($(HAS_LIBDRM),y)
-	LIBS += $(shell $(PKG_CONFIG) --libs libdrm)
-	CFLAGS_EXTRA += $(shell $(PKG_CONFIG) --cflags libdrm)
-else
-	OUTS := $(filter-out libdrm_%$(OUT_EXT),$(OUTS))
-endif
-ifeq ($(HAS_OPENBLAS),y)
-	LIBS += $(shell $(PKG_CONFIG) --libs openblas)
-	CFLAGS_EXTRA += $(shell $(PKG_CONFIG) --cflags openblas)
-else
-	OUTS := $(filter-out openblas_%$(OUT_EXT),$(OUTS))
-endif
-OUTS := $(addprefix $(OUT_DIR)/,$(OUTS))
-
-all: mkdir $(OUTS)
-	for subdir in $(SUBDIRS); do \
-	  if [ -d "$${subdir}" ]; then \
-	    $(MAKE) -C "$${subdir}" OUT_DIR="$(OUT_DIR)/$$subdir"; \
-	  fi \
-	done
-
-$(COMMON_OBJ): $(COMMON_DIR)/$(COMMON_BASENAME)$(IN_EXT_C)
-	$(CC) $(CFLAGS) -c -o '$@' '$<' $(LIBS)
-
-$(OUT_DIR)/%$(OUT_EXT): %$(IN_EXT_ASM) $(COMMON_OBJ)
-	$(CC) $(CFLAGS) $(COMMON_OBJ) -o '$@' '$<' $(LIBS)
-
-$(OUT_DIR)/%$(OUT_EXT): %$(IN_EXT_C) $(COMMON_OBJ)
-	$(CC) $(CFLAGS) $(COMMON_OBJ) -o '$@' '$<' $(LIBS)
-
-$(OUT_DIR)/%$(OUT_EXT): %$(IN_EXT_CXX) $(COMMON_OBJ)
-	$(CXX) $(CXXFLAGS) -o '$@' '$<' $(LIBS)
-
-clean:
-	rm -f *'$(OBJ_EXT)' *'$(OUT_EXT)'
-	for subdir in $(SUBDIRS); do \
-	  if [ -d "$${subdir}" ]; then \
-	    $(MAKE) -C $${subdir} clean; \
-	  fi \
-	done
-
-mkdir:
-	mkdir -p '$(OUT_DIR)'