Implement dladdr() (partially)

Note that this always returns with dli_sname and dli_saddr set to NULL,
indicating no symbol matching addr could be found.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>

winsup/cygwin/include/dlfcn.h

@@ -9,6 +9,9 @@ details. */
 #ifndef _DLFCN_H
 #define _DLFCN_H
 
+#include <sys/cdefs.h>
+#include <limits.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -42,6 +45,21 @@ extern void dlfork (int);
 #define RTLD_DEEPBIND  32	/* Place lookup scope so that this lib is    */
 				/* preferred over global scope.  */
 
+
+#if __GNU_VISIBLE
+typedef struct Dl_info Dl_info;
+
+struct Dl_info
+{
+   char        dli_fname[PATH_MAX];  /* Filename of defining object */
+   void       *dli_fbase;            /* Load address of that object */
+   const char *dli_sname;            /* Name of nearest lower symbol */
+   void       *dli_saddr;            /* Exact value of nearest symbol */
+};
+
+extern int dladdr (const void *addr, Dl_info *info);
+#endif
+
 #ifdef __cplusplus
 }
 #endif