Fix issue #3229, refs #3184

src/Composer/Installer/LibraryInstaller.php

@@ -316,7 +316,7 @@ class LibraryInstaller implements InstallerInterface
         $proxyCode = <<<PROXY
 #!/usr/bin/env sh
 
-dir=$(d=$(dirname "$0"); cd "\$d"; cd $binDir && pwd)
+dir=$(d=\${0%[/\\\\]*}; cd "\$d"; cd $binDir && pwd)
 
 # See if we are running in Cygwin by checking for cygpath program
 if command -v 'cygpath' >/dev/null 2>&1; then