glcpp: Add support for lexing from a string.

The standalone binary still reads from stdin, however.

glcpp/glcpp-lex.l

@@ -201,3 +201,9 @@ NON_STARS_THEN_STARS	[^*]*[*]+
 }
 
 %%
+
+void
+glcpp_lex_set_source_string(glcpp_parser_t *parser, const char *shader)
+{
+	yy_scan_string(shader, parser->scanner);
+}

glcpp/glcpp.h

@@ -159,6 +159,9 @@ glcpp_parser_destroy (glcpp_parser_t *parser);
 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);