glcpp: Add support for lexing from a string.

The standalone binary still reads from stdin, however.
diff --git a/glcpp/glcpp.h b/glcpp/glcpp.h
index ac103b7..0d43f0b 100644
--- a/glcpp/glcpp.h
+++ b/glcpp/glcpp.h
@@ -159,6 +159,9 @@
 int
 glcpp_lex_init_extra (glcpp_parser_t *parser, yyscan_t* scanner);
 
+void
+glcpp_lex_set_source_string(glcpp_parser_t *parser, const char *shader);
+
 int
 glcpp_lex (yyscan_t scanner);