imap-send: handle NO_OPENSSL even when openssl exists

If NO_OPENSSL is defined, then imap-send.c defines a fallback "SSL"
type, which is just a void pointer that remains NULL. This works, but it
has one problem: it is using the type name "SSL", which conflicts with
the upstream name, if some other part of the system happens to include
openssl. For example:

  $ make NO_OPENSSL=Nope OPENSSL_SHA1=Yes imap-send.o
      CC imap-send.o
  imap-send.c:35:15: error: conflicting types for ‘SSL’; have ‘void *’
     35 | typedef void *SSL;
        |               ^~~
  In file included from /usr/include/openssl/evp.h:26,
                   from sha1/openssl.h:4,
                   from hash.h:10,
                   from object.h:4,
                   from commit.h:4,
                   from refs.h:4,
                   from setup.h:4,
                   from imap-send.c:32:
  /usr/include/openssl/types.h:187:23: note: previous declaration of ‘SSL’ with type ‘SSL’ {aka ‘struct ssl_st’}
    187 | typedef struct ssl_st SSL;
        |                       ^~~
  make: *** [Makefile:2761: imap-send.o] Error 1

This is not a terribly common combination in practice:

  1. Why are we disabling openssl support but still using its sha1? The
     answer is that you may use the same build options across many
     versions, and some older versions of Git no longer build with
     modern versions of openssl.

  2. Why are we using a totally unsafe sha1 that does not detect
     collisions? You're right, we shouldn't. But in preparation for
     using unsafe sha1 for non-cryptographic checksums, it would be nice
     to be able to turn it on without hassle.

We can make this work by adjusting the way imap-send handles its
fallback. One solution is something like this:

  #ifdef NO_OPENSSL
  #define git_SSL void *
  #else
  #define git_SSL SSL
  #endif

But we can observe that we only need this definition in one spot: the
struct which holds the variable. So rather than play around with macros
that may cause unexpected effects, we can just directly use the correct
type in that struct.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

imap-send.c

@@ -29,9 +29,6 @@
 #include "parse-options.h"
 #include "setup.h"
 #include "strbuf.h"
-#if defined(NO_OPENSSL) && !defined(HAVE_OPENSSL_CSPRNG)
-typedef void *SSL;
-#endif
 #ifdef USE_CURL_FOR_IMAP_SEND
 #include "http.h"
 #endif
@@ -83,7 +80,11 @@ struct imap_server_conf {
 
 struct imap_socket {
 	int fd[2];
+#if defined(NO_OPENSSL) && !defined(HAVE_OPENSSL_CSPRNG)
+	void *ssl;
+#else
 	SSL *ssl;
+#endif
 };
 
 struct imap_buffer {