add build_all.sh to allow 1-liner to build development version of the compiler (#8546)

build_all.sh

@@ -0,0 +1,22 @@
+#! /bin/sh
+
+# build development version of the compiler; can be rerun safely
+
+set -u # error on undefined variables
+set -e # exit on first error
+
+echo_run(){
+  echo "\n$@"
+  "$@"
+}
+
+[ -d csources ] || echo_run git clone --depth 1 https://github.com/nim-lang/csources.git
+(
+  ## avoid changing dir in case of failure
+  echo_run cd csources
+  echo_run sh build.sh
+)
+
+echo_run bin/nim c koch
+echo_run ./koch boot -d:release
+echo_run ./koch tools # Compile Nimble and other tools.