cmake: fix ENABLE_WERROR=ON breaking auto-detections

- cmake: fix compiler warnings in `CheckNonblockingSocketSupport`.
  detection functions.

  Without this, these detections fail when `ENABLE_WERROR=ON`.

- cmake: disable ENABLE_WERROR for MSVC during symbol checks in `src`.

  CMake's built-in symbol check function `check_symbol_exists()`
  generate warnings with MSVC. With warnings considered errors, these
  detections fail permanently. Our workaround is to disable
  warnings-as-errors while running these checks.

  ```
  CheckSymbolExists.c(8): warning C4054: 'type cast': from function pointer '__int64 (__cdecl *)(const char *,char **,int)' to data pointer 'int *'
  in `return ((int*)(&strtoll))[argc];`
  ```

  Ref: https://ci.appveyor.com/project/libssh2org/libssh2/builds/46537222/job/4vg4yg333mu2lg9b

- example: replace `strcasecmp()` with C89 `strcmp()`.

  To avoid using CMake symbol checks in `example`.

  Another option is to duplicate the `check_symbol_exists()` workaround
  from `src`, but I figure it's not worth the complexity. We use
  `strcasecmp()` solely to check optional command-line options for
  example programs, and those are fine as lower-case.

  Without this, these detections fail when `ENABLE_WERROR=ON`.

- also delete `__function__` detection/use in `example`.

  To avoid the complexity for the sake of using it at a single place in
  of the example's error branch. Replace that use with a literal name of
  the function.

- cmake: also use `CMakePushCheckState` functions instead of manual
  save/restore.

Closes #857

cmake/CheckNonblockingSocketSupport.cmake

@@ -47,10 +47,10 @@ macro(check_nonblocking_socket_support)
 #error \"O_NONBLOCK does not work on this platform\"
 #endif
 
-int main()
+int main(void)
 {
-    int socket;
-    int flags = fcntl(socket, F_SETFL, flags | O_NONBLOCK);
+    int socket = 0;
+    (void)fcntl(socket, F_SETFL, O_NONBLOCK);
 }"
   HAVE_O_NONBLOCK)
 
@@ -59,10 +59,11 @@ int main()
 #include <unistd.h>
 #include <stropts.h>
 
-int main()
+int main(void)
 {
-    int socket;
-    int flags = ioctl(socket, FIONBIO, &flags);
+    int socket = 0;
+    int flags = 0;
+    (void)ioctl(socket, FIONBIO, &flags);
 }"
     HAVE_FIONBIO)
 
@@ -76,12 +77,11 @@ int main()
 #include <windows.h>
 #include <winsock2.h>
 
-int main()
+int main(void)
 {
-    SOCKET sd;
+    SOCKET sd = socket(0, 0, 0);
     unsigned long flags = 0;
-    sd = socket(0, 0, 0);
-    ioctlsocket(sd, FIONBIO, &flags);
+    (void)ioctlsocket(sd, FIONBIO, &flags);
 }"
       HAVE_IOCTLSOCKET)
 
@@ -89,10 +89,10 @@ int main()
         check_c_source_compiles("/* IoctlSocket test (Amiga?) */
 #include <sys/ioctl.h>
 
-int main()
+int main(void)
 {
-    int socket;
-    int flags = IoctlSocket(socket, FIONBIO, (long)1);
+    int socket = 0;
+    (void)IoctlSocket(socket, FIONBIO, (long)1);
 }"
         HAVE_IOCTLSOCKET_CASE)
 
@@ -100,11 +100,11 @@ int main()
           check_c_source_compiles("/* SO_NONBLOCK test (BeOS) */
 #include <socket.h>
 
-int main()
+int main(void)
 {
     long b = 1;
-    int socket;
-    int flags = setsockopt(socket, SOL_SOCKET, SO_NONBLOCK, &b, sizeof(b));
+    int socket = 0;
+    (void)setsockopt(socket, SOL_SOCKET, SO_NONBLOCK, &b, sizeof(b));
 }"
           HAVE_SO_NONBLOCK)