symlinks for empty tiles not supported on Windows

[geographika]

It is a known issue that MapCache on Windows does not support <symlink_blank/>

This issue gathers some links and a possible approach to implementing this functionality.

The Windows API function CreateSymbolicLinkA should provide the equivalent of symlink.

See https://stackoverflow.com/questions/6472913/porting-unix-code-with-symlink-function-to-windows - possible permissions issues may however occur.

Links to codebases where this is implemented:

• https://github.com/jpembedded/vuhol_minix/blob/ee6dce2d2d4f2bce831fc198acc625a2b17e1a29/external/bsd/libarchive/dist/libarchive/test/main.c#L121
• https://github.com/gavoski/audacity/blob/08ce17fa686b1c04edcf9797b01e94abcc3aa1d1/lib-src/lv2/windows/lilv_config.h
• https://github.com/rmyorston/busybox-w32/commit/375cda9a88024135d630ca8990d9aff4ea414e89