warn if "make update" was not executed

and warn if Make's parallelism is not enabled

README.md

@@ -144,6 +144,18 @@ Link PCRE, defaults to 1.
 
 Internal target that creates the directory with the same name.
 
+### sanity-checks
+
+Internal target used to check that a C compiler is installed.
+
+### warn-update
+
+Internal target that checks if `make update` was executed for the current Git commit.
+
+### warn-jobs
+
+Internal target that checks if Make's parallelism was enabled by specifying the number of jobs.
+
 ### deps-common
 
 Internal target that needs to be a dependency for a custom "deps" target which,

makefiles/targets.mk

@@ -7,6 +7,8 @@
 
 .PHONY: \
 	sanity-checks \
+	warn-update \
+	warn-jobs \
 	deps-common \
 	build-nim \
 	update-common \
@@ -36,9 +38,24 @@ build:
 sanity-checks:
 	which $(CC) &>/dev/null || { echo "C compiler ($(CC)) not installed. Aborting."; exit 1; }
 
-#- runs only the first time and after `make update`, so have "normal"
+#- check if "make update" was executed
-#  (timestamp-checked) prerequisites here
+warn-update:
-deps-common: sanity-checks $(NIMBLE_DIR)
+	if [[ -e $(UPDATE_TIMESTAMP) ]]; then \
+		if [[ $$(cat $(UPDATE_TIMESTAMP)) -ne $$($(GET_CURRENT_COMMIT_TIMESTAMP)) ]]; then \
+			echo -e "\n\"$$(basename "$(MAKE)") update\" was not executed for the current Git commit! The resulting build is unsupported.\n"; \
+		fi; \
+	fi
+
+#- check if we're enabling multiple jobs - https://stackoverflow.com/a/48865939
+warn-jobs:
+	+ if [[ ! "${MAKEFLAGS}" =~ --jobserver[^=]+= ]]; then \
+		NPROC=$$($(NPROC_CMD)); \
+		if [[ $${NPROC} -gt 1 ]]; then \
+			echo -e "\nTip of the day: this will probably build faster if you use \"$$(basename "$(MAKE)") -j$${NPROC} ...\".\n"; \
+		fi; \
+	fi
+
+deps-common: | sanity-checks warn-update warn-jobs $(NIMBLE_DIR)
 # - don't build our Nim target if it's not going to be used
 ifeq ($(USE_SYSTEM_NIM), 0)
 deps-common: $(NIM_BINARY)
@@ -66,6 +83,7 @@ build-nim: | sanity-checks
 #- initialises and updates the Git submodules
 #- hard-resets the working copies of submodules
 #- deletes "nimcache" directories
+#- updates ".update.timestamp"
 #- deletes the ".nimble" dir and executes the "deps" target
 #- allows parallel building with the '+' prefix
 #- rebuilds the Nim compiler if the corresponding submodule is updated
@@ -77,6 +95,7 @@ update-common: | sanity-checks
 	git submodule update --init --recursive
 	git submodule foreach --quiet --recursive 'git reset --quiet --hard'
 	find . -type d -name nimcache -print0 | xargs -0 rm -rf
+	$(GET_CURRENT_COMMIT_TIMESTAMP) > $(UPDATE_TIMESTAMP)
 	rm -rf $(NIMBLE_DIR)
 	+ "$(MAKE)" --no-print-directory deps
 ifeq ($(USE_SYSTEM_NIM), 0)

makefiles/variables.mk

@@ -66,33 +66,46 @@ GIT_STATUS := git status
 #- duplicated in "env.sh" for the env var with the same name
 NIMBLE_DIR := vendor/.nimble
 REPOS_DIR := vendor
+
 ifeq ($(OS), Windows_NT)
   PWD := pwd -W
+  EXE_SUFFIX := .exe
+  # available since Git 2.9.4
+  GIT_TIMESTAMP_ARG := --date=unix
 else
   PWD := pwd
+  EXE_SUFFIX :=
+  # available since Git 2.7.0
+  GIT_TIMESTAMP_ARG := --date=format-local:%s
 endif
+
+ifeq ($(shell uname), Darwin)
+  # md5sum - macOS is a special case
+  MD5SUM := md5 -r
+  NPROC_CMD := sysctl -n hw.logicalcpu
+else
+  MD5SUM := md5sum
+  NPROC_CMD := nproc
+endif
+
+GET_CURRENT_COMMIT_TIMESTAMP := git log --pretty=format:%cd -n 1 $(GIT_TIMESTAMP_ARG)
+UPDATE_TIMESTAMP := .update.timestamp
+
 ifeq ($(BUILD_SYSTEM_DIR),)
   $(error You need to define BUILD_SYSTEM_DIR before including this file)
 endif
+
 # we want a "recursively expanded" (delayed interpolation) variable here, so we can set CMD in rule recipes
 RUN_CMD_IN_ALL_REPOS = git submodule foreach --recursive --quiet 'echo -e "\n\e[32m$$name:\e[39m"; $(CMD)'; echo -e "\n\e[32m$$($(PWD)):\e[39m"; $(CMD)
+
 # absolute path, since it will be run at various subdirectory depths
 ENV_SCRIPT := "$(CURDIR)/$(BUILD_SYSTEM_DIR)/scripts/env.sh"
+
 # duplicated in "env.sh" to prepend NIM_DIR/bin to PATH
 NIM_DIR := $(BUILD_SYSTEM_DIR)/vendor/Nim
 
-ifeq ($(OS), Windows_NT)
-  EXE_SUFFIX := .exe
-else
-  EXE_SUFFIX :=
-endif
 NIM_BINARY := $(NIM_DIR)/bin/nim$(EXE_SUFFIX)
-# md5sum - macOS is a special case
+
-ifeq ($(shell uname), Darwin)
-  MD5SUM := md5 -r
-else
-  MD5SUM := md5sum
-endif
 # AppVeyor/Travis cache of $(NIM_DIR)/bin
 CI_CACHE :=