sim: support dlopen in -lc

Stop assuming that dlopen is only available via -ldl.  Newer versions
of glibc have merged it into -lc which broke this configure test.

sim/configure

@@ -12915,7 +12915,7 @@ if test "$ac_res" != no; then :
 
 fi
 
-if test "${ac_cv_lib_dl_dlopen}" = "yes"; then
+if test "${ac_cv_search_dlopen}" = "none required" || test "${ac_cv_lib_dl_dlopen}" = "yes"; then
 
 pkg_failed=no
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for sdl2" >&5

sim/m4/sim_ac_platform.m4

@@ -151,7 +151,7 @@ AC_SEARCH_LIBS([fabs], [m])
 AC_SEARCH_LIBS([log2], [m])
 
 AC_SEARCH_LIBS([dlopen], [dl])
-if test "${ac_cv_lib_dl_dlopen}" = "yes"; then
+if test "${ac_cv_search_dlopen}" = "none required" || test "${ac_cv_lib_dl_dlopen}" = "yes"; then
   PKG_CHECK_MODULES(SDL, sdl2, [dnl
     SDL_CFLAGS="${SDL_CFLAGS} -DHAVE_SDL=2"
   ], [