When creating a unified fasl, the init function name has to be exported outside the DLL. In cygwin this means adding ECL_DLLEXPORT to +lisp-program-init+.

src/cmp/cmpmain.lsp

@@ -230,6 +230,7 @@ the environment variable TMPDIR to a different value." template)))
 #ifdef __cplusplus
 extern \"C\"
 #endif
+ECL_DLLEXPORT
 void ~A(cl_object cblock)
 {
 	static cl_object Cblock;