#1574 Loading of external lexers broken in 3.3.8

[Markus Nißl]

In 3.3.8, the constructor of ExternalLexerModule only stores a pointer to the string holding the name of the external lexer.

The pointer to that string is a local variable in a loop in LexerLibrary::LexerLibrary().

The string is destroyed ... and eventually the external lexer cannot be loaded anymore :-(

[Neil Hodgson]

Fix committed as [1fab94].