\version "2.16.0"

includeIfExists =
#(define-music-function (parser location file)(string?)
   (if (file-exists? file)
       (ly:parser-include-string parser (format "\\include \"~A\"\n" file)))
   (make-music 'SequentialMusic 'void #t))

\includeIfExists "config.ly"

{ c''4 }