dlfcn: always define _GNU_SOURCE

We need this for the freebsd kernel with glibc as used in the Debian kfreebsd
ports.  There shouldn't be a problem defining this on systems not using glibc.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/dso/dso_dlfcn.c

@@ -60,10 +60,8 @@
    that handle _GNU_SOURCE and other similar macros.  Defining it later
    is simply too late, because those headers are protected from re-
    inclusion.  */
-#ifdef __linux
-# ifndef _GNU_SOURCE
-#  define _GNU_SOURCE	/* make sure dladdr is declared */
-# endif
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE	/* make sure dladdr is declared */
 #endif
 
 #include <stdio.h>