autoconf fixes and cleanups (check for dlopen() in libc too).

diff --git a/include/elfdll.h b/include/elfdll.h
index a8c5c10..94bfef8 100644
--- a/include/elfdll.h
+++ b/include/elfdll.h
@@ -5,7 +5,7 @@
 HINSTANCE16 ELFDLL_LoadModule16(LPCSTR libname, BOOL implicit);
 void ELFDLL_UnloadLibrary(WINE_MODREF *wm);
 
-#if defined(HAVE_LIBDL) && defined(HAVE_DLFCN_H)
+#if defined(HAVE_DL_API)
 
 void *ELFDLL_dlopen(const char *libname, int flags);
 extern char *extra_ld_library_path;