Extend test-dlopen-so to also cover cases when built without support

Let's make things more consistent and have all dlopen_xxx() functions
return EOPNOTSUPP on failure and verify this behavior in test-dlopen-so.

src/basic/gcrypt-util.h

@@ -4,6 +4,10 @@
 
 #include "forward.h"
 
+int dlopen_gcrypt(void);
+
+int initialize_libgcrypt(bool secmem);
+
 #if HAVE_GCRYPT
 #include <gcrypt.h> /* IWYU pragma: export */
 
@@ -40,8 +44,6 @@ extern DLSYM_PROTOTYPE(gcry_prime_check);
 extern DLSYM_PROTOTYPE(gcry_randomize);
 extern DLSYM_PROTOTYPE(gcry_strerror);
 
-int initialize_libgcrypt(bool secmem);
-
 /* Copied from gcry_md_putc from gcrypt.h due to the need to call the sym_ variant */
 #define sym_gcry_md_putc(h,c)                              \
         do {                                               \