Standard make path (#1353)

Modifies download-cosmocc.sh to maintain a .cosmocc/current symlink that
always points to the most recently downloaded version of cosmocc. We can
use this to point at a canonical make for a bootstrapped repository. For
first-time builds, we suggest: https://cosmo.zip/pub/cosmos/bin/make and
have updated the docs in a few places to mention this.

Fixes the other part of #1346.

tool/zsh/mmake

@@ -38,8 +38,21 @@ done
   whence nproc >/dev/null || autoload -Uz nproc
   j=-j$(nproc)
 }
-local make=${MAKE:-${COSMOCC:-/opt/cosmocc/current}/bin/make}
-[[ -x $make ]] || make=${COSMO:-$PWD}/build/bootstrap/make
+local make=$(
+  case $MAKE in
+    */*) echo $MAKE ;;
+    ?*)  command -v $MAKE ;;
+    *)   echo .cosmocc/current/bin/make
+  esac
+)
+if [[ ! -x $make ]]; then
+  { echo 'please install a suitable make, for example:'
+    echo
+    echo 'https://cosmo.zip/pub/cosmos/bin/make'
+    echo
+    echo 'then either put it on your $PATH or point to it with $MAKE.'
+  } >&2; return 1
+fi
 ( set -x
   exec $make $j $flags MODE=$mode $targs )
 # vim:ft=zsh