Fix the MS-Windows MinGW build

Fix the MS-Windows MinGW build

* build_w32.bat (GccCompile): Use -std=gnu99, as some code uses
C99 features ('for' loop initial declarations).

* dir.c (print_dir_data_base) [WINDOWS32]: Cast 'mtime' to
'int64_t', and use %I64d to print it, to avoid compile-time
warning about printing a 'time_t' value, which could be either
a 32-bit or a 64 bit integral type.