diff --git a/Makefile b/Makefile
index ac77b8c79..d9d73402b 100644
--- a/Makefile
+++ b/Makefile
@@ -34,44 +34,14 @@ NIM_DIR := vendor/Nim
 #  but this is broken when using symlinks, so build csources separately (we get parallel compiling as a bonus)
 #- Windows is a special case, as usual
 #- macOS is also a special case, with its "ln" not supporting "-r"
+#- the AppVeyor 32-build is done on a 64-bit image, so we need to override the architecture detection with ARCH_OVERRIDE
+BUILD_NIM := echo -e $(BUILD_MSG) "Nim compiler" && \
+	V=$(V) CC=$(CC) MAKE=$(MAKE) ARCH_OVERRIDE=$(ARCH_OVERRIDE) "$(CURDIR)/build_nim.sh" "$(NIM_DIR)" ../Nim-csources ../nimble
 ifeq ($(OS), Windows_NT)
-  # the AppVeyor 32-build is done on a 64-bit image, so we need to override the architecture detection
-  ifeq ($(ARCH_OVERRIDE), x86)
-    UCPU := ucpu=i686
-  else
-    UCPU :=
-  endif
-
-  BUILD_CSOURCES := \
-    $(MAKE) myos=windows $(UCPU) clean $(HANDLE_OUTPUT) && \
-    $(MAKE) myos=windows $(UCPU) CC=gcc LD=gcc $(HANDLE_OUTPUT)
   EXE_SUFFIX := .exe
 else
-  BUILD_CSOURCES := \
-    $(MAKE) clean $(HANDLE_OUTPUT) && \
-    $(MAKE) LD=$(CC) $(HANDLE_OUTPUT)
   EXE_SUFFIX :=
 endif
-BUILD_NIM := echo -e $(BUILD_MSG) "Nim compiler" && \
-	cd $(NIM_DIR) && \
-	rm -rf bin/nim_csources csources dist/nimble && \
-	ln -s ../Nim-csources csources && \
-	mkdir -p dist && \
-	ln -s ../../nimble dist/nimble && \
-	cd csources && \
-	$(BUILD_CSOURCES) && \
-	cd - >/dev/null && \
-	[ -e csources/bin ] && { \
-		cp -a csources/bin/nim bin/nim && \
-		cp -a csources/bin/nim bin/nim_csources && \
-		rm -rf csources/bin; \
-	} || { \
-		cp -a bin/nim bin/nim_csources; \
-	} && { \
-		sed 's/koch tools/koch --stable tools/' build_all.sh > build_all_custom.sh; \
-		sh build_all_custom.sh $(HANDLE_OUTPUT); \
-		rm build_all_custom.sh; \
-	} && cd ../..
 NIM_BINARY := $(NIM_DIR)/bin/nim$(EXE_SUFFIX)
 # md5sum - macOS is a special case
 ifeq ($(shell uname), Darwin)
@@ -197,10 +167,20 @@ build-nim: | deps
 #- rebuilds the Nim compiler after the corresponding submodule is updated (keep in mind that Git doesn't preserve file timestamps)
 $(NIM_BINARY) update: | sanity-checks
 	git submodule update --init --recursive
-	[[ -n "$(CI_CACHE)" && -d "$(CI_CACHE)" ]] && cp -a "$(CI_CACHE)"/* $(NIM_DIR)/bin/ || true
-	rm -rf $(NIMBLE_DIR) nimbus.nims && $(MAKE) nimbus.nims
-	+ [[ -e $(NIM_BINARY) &&  $$(stat -c%Z $(NIM_BINARY)) -gt $$(git log --pretty=format:%cd -n 1 --date=unix --  vendor/Nim) ]] || \
-		{ $(BUILD_NIM); [[ -n "$(CI_CACHE)" ]] && rm -rf "$(CI_CACHE)" && mkdir $(CI_CACHE) && cp -a $(NIM_DIR)/bin/* "$(CI_CACHE)"/ || true; }
+	[[ -n "$(CI_CACHE)" && -d "$(CI_CACHE)" ]] && \
+		cp -a "$(CI_CACHE)"/* $(NIM_DIR)/bin/ || \
+		true
+	rm -rf $(NIMBLE_DIR) nimbus.nims && \
+		$(MAKE) nimbus.nims
+	+ "$(CURDIR)/nim_needs_rebuilding.sh" "$(NIM_DIR)" $(NIM_BINARY) && \
+		{ \
+			$(BUILD_NIM); \
+			[[ -n "$(CI_CACHE)" ]] && \
+			rm -rf "$(CI_CACHE)" && \
+			mkdir $(CI_CACHE) && \
+			cp -a $(NIM_DIR)/bin/* "$(CI_CACHE)"/ || \
+			true; \
+		} || true
 
 # don't use this target, or you risk updating dependency repos that are not ready to be used in Nimbus
 update-remote:
diff --git a/build_nim.sh b/build_nim.sh
new file mode 100755
index 000000000..1bc287be9
--- /dev/null
+++ b/build_nim.sh
@@ -0,0 +1,83 @@
+#!/bin/bash
+
+set -e
+
+# Git commits
+CSOURCES_COMMIT="b56e49bbedf62db22eb26388f98262e2948b2cbc" # 0.19.0
+NIMBLE_COMMIT="c8d79fc0228682677330a9f57d14389aaa641153" # Mar 26 10:06:06 2019
+
+# script arguments
+[[ $# -ne 3 ]] && { echo "usage: $0 nim_dir csources_dir nimble_dir"; exit 1; }
+NIM_DIR="$1"
+CSOURCES_DIR="$2"
+NIMBLE_DIR="$3"
+
+## env vars
+# verbosity level
+[[ -z "$V" ]] && V=0
+[[ "$V" == "0" ]] && exec &>/dev/null
+[[ -z "$CC" ]] && CC="gcc"
+# to build csources in parallel, set MAKE="make -jN"
+[[ -z "$MAKE" ]] && MAKE="make"
+# for 32-bit binaries on a 64-bit Windows host
+UCPU=""
+[[ "$ARCH_OVERRIDE" == "x86" ]] && UCPU="ucpu=i686"
+
+# Windows detection
+ON_WINDOWS=0
+uname | grep -qi mingw && ON_WINDOWS=1
+
+# working directory
+cd "$NIM_DIR"
+
+# Git repos for csources and Nimble
+[[ -d "$CSOURCES_DIR" ]] || { \
+	mkdir -p "$CSOURCES_DIR" && \
+	cd "$CSOURCES_DIR" && \
+	git clone https://github.com/nim-lang/csources.git . && \
+	git checkout $CSOURCES_COMMIT && \
+	cd - >/dev/null; \
+}
+[[ "$CSOURCES_DIR" != "csources" ]] && \
+rm -rf csources && \
+ln -s "$CSOURCES_DIR" csources
+
+# we have to delete .git or koch.nim will checkout a branch tip
+[[ -d "$NIMBLE_DIR" ]] || { \
+	mkdir -p "$NIMBLE_DIR" && \
+	cd "$NIMBLE_DIR" && \
+	git clone https://github.com/nim-lang/nimble.git . && \
+	git checkout $NIMBLE_COMMIT && \
+	rm -rf .git && \
+	cd - >/dev/null; \
+}
+[[ "$NIMBLE_DIR" != "dist/nimble" ]] && \
+mkdir -p dist && \
+rm -rf dist/nimble && \
+ln -s ../"$NIMBLE_DIR" dist/nimble
+
+# bootstrap the Nim compiler and build the tools
+rm -rf bin/nim_csources && \
+cd csources && { \
+	[[ "$ON_WINDOWS" == "0" ]] && { \
+		$MAKE clean && \
+		$MAKE LD=$CC; \
+	} || { \
+		$MAKE myos=windows $UCPU clean && \
+		$MAKE myos=windows $UCPU CC=gcc LD=gcc; \
+	}; \
+} && \
+cd - >/dev/null && { \
+	[ -e csources/bin ] && { \
+		cp -a csources/bin/nim bin/nim && \
+		cp -a csources/bin/nim bin/nim_csources && \
+		rm -rf csources/bin; \
+	} || { \
+		cp -a bin/nim bin/nim_csources; \
+	}; \
+} && { \
+	sed 's/koch tools/koch --stable tools/' build_all.sh > build_all_custom.sh; \
+	sh build_all_custom.sh; \
+	rm build_all_custom.sh; \
+}
+
diff --git a/nim_needs_rebuilding.sh b/nim_needs_rebuilding.sh
new file mode 100755
index 000000000..a1553597b
--- /dev/null
+++ b/nim_needs_rebuilding.sh
@@ -0,0 +1,13 @@
+#!/bin/bash
+
+# this one is required
+set -e
+
+# script arguments
+[[ $# -ne 2 ]] && { echo "usage: $0 nim_dir nim_binary"; exit 1; }
+NIM_DIR="$1"
+NIM_BINARY="$2"
+
+# compare binary mtime to the date of the last commit
+! [[ -e $NIM_BINARY && $(stat -c%Y $NIM_BINARY) -gt $(cd "$NIM_DIR"; git log --pretty=format:%cd -n 1 --date=unix) ]]
+