build: speed up and extend picky compiler options

Implement picky warnings with clang in autotools. Extend picky gcc
warnings, sync them between build tools and compilers and greatly
speed up detection in CMake.

- autotools: enable clang compiler warnings with `--enable-debug`.

- autotools: enable more gcc compiler warnings with `--enable-debug`.

- autotools/cmake: sync compiler warning options between gcc and clang.

- sync compiler warning options between autotools and cmake.

- cmake: reduce option-checks to speed up the detection phase.
  Bring them down to 3 (from 35). Leaving some checks to keep the
  CMake logic alive and for an easy way to add new options.

  clang 3.0 (2011-11-29) and gcc 2.95 (1999-07-31) now required.

- autotools logic copied from curl, with these differences:

  - delete `-Wimplicit-fallthrough=4` due to a false positive.

  - reduce `-Wformat-truncation=2` to `1` due to a false positive.

  - simplify MinGW detection for `-Wno-pedantic-ms-format`.

- cmake: show enabled picky compiler options (like autotools).

- cmake: do compile `tests/simple.c` and `tests/ssh2.c`.

- fix new compiler warnings.

- `tests/CMakeLists.txt`: fix indentation.

Original source of autotools logic:
- a8fbdb461c/acinclude.m4
- a8fbdb461c/m4/curl-compilers.m4

Notice that the autotools implementation considers Apple clang as
legacy clang 3.7. CMake detection works more accurately, at the same
time more error-prone and difficult to update due to the sparsely
documented nature of Apple clang option evolution.

Closes #952

src/agent.c

@@ -128,20 +128,22 @@ agent_connect_unix(LIBSSH2_AGENT *agent)
 }
 
 #define RECV_SEND_ALL(func, socket, buffer, length, flags, abstract) \
-    size_t finished = 0;                                             \
+    do {                                                             \
+        size_t finished = 0;                                         \
                                                                      \
-    while(finished < length) {                                       \
-        ssize_t rc;                                                  \
-        rc = func(socket,                                            \
-                  (char *)buffer + finished, length - finished,      \
-                  flags, abstract);                                  \
-        if(rc < 0)                                                   \
-            return rc;                                               \
+        while(finished < length) {                                   \
+            ssize_t rc;                                              \
+            rc = func(socket,                                        \
+                      (char *)buffer + finished, length - finished,  \
+                      flags, abstract);                              \
+            if(rc < 0)                                               \
+                return rc;                                           \
                                                                      \
-        finished += rc;                                              \
-    }                                                                \
+            finished += rc;                                          \
+        }                                                            \
                                                                      \
-    return finished;
+        return finished;                                             \
+    } while(0)
 
 static ssize_t _send_all(LIBSSH2_SEND_FUNC(func), libssh2_socket_t socket,
                          const void *buffer, size_t length,
@@ -242,7 +244,7 @@ agent_disconnect_unix(LIBSSH2_AGENT *agent)
     return LIBSSH2_ERROR_NONE;
 }
 
-struct agent_ops agent_ops_unix = {
+static struct agent_ops agent_ops_unix = {
     agent_connect_unix,
     agent_transact_unix,
     agent_disconnect_unix
@@ -347,7 +349,7 @@ agent_disconnect_pageant(LIBSSH2_AGENT *agent)
     return 0;
 }
 
-struct agent_ops agent_ops_pageant = {
+static struct agent_ops agent_ops_pageant = {
     agent_connect_pageant,
     agent_transact_pageant,
     agent_disconnect_pageant