windows: search for a user-installed npm

This changes npm.cmd and the npm script when running in Cygwin to look
for an npm installation in prefix, running it if found. The default npm
is used to get the prefix. This implements the changes described in
nodejs/node-v0.x-archive#8528 .

Fixes nodejs/node-v0.x-archive#8528. Reference: nodejs/node-v0.x-archive#8554.

Fixes: npm#6412

PR-URL: npm/npm#9089

bin/npm

@@ -7,8 +7,21 @@ case `uname` in
     *CYGWIN*) basedir=`cygpath -w "$basedir"`;;
 esac
 
-if [ -x "$basedir/node.exe" ]; then
-  "$basedir/node.exe" "$basedir/node_modules/npm/bin/npm-cli.js" "$@"
-else
-  node "$basedir/node_modules/npm/bin/npm-cli.js" "$@"
+NODE_EXE="$basedir/node.exe"
+if ! [ -x "$NODE_EXE" ]; then
+  NODE_EXE=node
 fi
+
+NPM_CLI_JS="$basedir/node_modules/npm/bin/npm-cli.js"
+
+case `uname` in
+  *CYGWIN*)
+    NPM_PREFIX=`"$NODE_EXE" "$NPM_CLI_JS" prefix -g`
+    NPM_PREFIX_NPM_CLI_JS="$NPM_PREFIX/node_modules/npm/bin/npm-cli.js"
+    if [ -f "$NPM_PREFIX_NPM_CLI_JS" ]; then
+      NPM_CLI_JS="$NPM_PREFIX_NPM_CLI_JS"
+    fi
+    ;;
+esac
+
+"$NODE_EXE" "$NPM_CLI_JS" "$@"

bin/npm.cmd

@@ -1,6 +1,19 @@
 :: Created by npm, please don't edit manually.
-@IF EXIST "%~dp0\node.exe" (
-  "%~dp0\node.exe" "%~dp0\.\node_modules\npm\bin\npm-cli.js" %*
-) ELSE (
-  node "%~dp0\.\node_modules\npm\bin\npm-cli.js" %*
+@ECHO OFF
+
+SETLOCAL
+
+SET "NODE_EXE=%~dp0\node.exe"
+IF NOT EXIST "%NODE_EXE%" (
+  SET "NODE_EXE=node"
 )
+
+SET "NPM_CLI_JS=%~dp0\node_modules\npm\bin\npm-cli.js"
+FOR /F "delims=" %%F IN ('CALL "%NODE_EXE%" "%NPM_CLI_JS%" prefix -g') DO (
+  SET "NPM_PREFIX_NPM_CLI_JS=%%F\node_modules\npm\bin\npm-cli.js"
+)
+IF EXIST "%NPM_PREFIX_NPM_CLI_JS%" (
+  SET "NPM_CLI_JS=%NPM_PREFIX_NPM_CLI_JS%"
+)
+
+"%NODE_EXE%" "%NPM_CLI_JS%" %*