Fix building with --disable-ssl

[sebastic]

The 4.1.3 build fails when configured with --disable-ssl.

X509_STORE_CTX is only available when HAVE_SSL is set:

In file included from ./nrpe.c:44:
../include/nrpe-ssl.h:47:50: error: unknown type name 'X509_STORE_CTX'
int ssl_verify_callback_common(int preverify_ok, X509_STORE_CTX * ctx, int is_invalid);
                                                 ^

And the use_ssl flag needs to be disabled when HAVE_SSL is not set:

Undefined symbols for architecture x86_64:
  "_use_ssl", referenced from:
      _process_arguments in nrpe-4fa9f6.o
      _handle_connection in nrpe-4fa9f6.o
      _read_packet in nrpe-4fa9f6.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)