build: Fix wrong symlink created for import library on MinGW and Cygwin

This is a cherry-pick of commit 0bb2bb50e36b8f8058c27446e934d9f425b8ffe4
from branch 'libpng18'

Resolves: https://github.com/pnggroup/libpng/issues/783
Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>
2 files changed