build: type mismatch warning from OPENSSL symbol

The project is configured to treat warnings as errors.  The following
warning is encountered when compiling on Visual Studio 2015 (when
generating the solution from cmake) and with OPENSSL_API_1_1 is
defined, causing the build to fail:

```
warning C4245: 'initializing': conversion from 'long' to 'unsigned
long', signed/unsigned mismatch.
```

This patch suppresses the warning by explicitly casting the symbol
from OPENSSL to the expected type.

Signed-off-by: Keith Holman <keith.holman@windriver.com>

src/civetweb.c

@@ -425,9 +425,9 @@ mg_static_assert(sizeof(size_t) == 4 || sizeof(size_t) == 8,
                  "size_t data type size check");
 
 #if defined(_WIN32) && !defined(__SYMBIAN32__) /* WINDOWS include block */
-#include <windows.h>
 #include <winsock2.h> /* DTL add for SO_EXCLUSIVE */
 #include <ws2tcpip.h>
+#include <windows.h>
 
 typedef const char *SOCK_OPT_TYPE;
 
@@ -5520,6 +5520,7 @@ kill(pid_t pid, int sig_num)
 #define WNOHANG (1)
 #endif
 
+#if defined(USE_TIMERS)
 pid_t
 waitpid(pid_t pid, int *status, int flags)
 {
@@ -5541,6 +5542,7 @@ waitpid(pid_t pid, int *status, int flags)
 	}
 	return (pid_t)-1;
 }
+#endif /* USE_TIMERS */
 
 
 static void
@@ -15401,7 +15403,7 @@ ssl_use_pem_file(struct mg_context *phys_ctx,
 static unsigned long
 ssl_get_protocol(int version_id)
 {
-	long unsigned ret = SSL_OP_ALL;
+	long unsigned ret = (long unsigned)SSL_OP_ALL;
 	if (version_id > 0)
 		ret |= SSL_OP_NO_SSLv2;
 	if (version_id > 1)
@@ -15416,7 +15418,7 @@ ssl_get_protocol(int version_id)
 static long
 ssl_get_protocol(int version_id)
 {
-	long ret = SSL_OP_ALL;
+	long ret = (long)SSL_OP_ALL;
 	if (version_id > 0)
 		ret |= SSL_OP_NO_SSLv2;
 	if (version_id > 1)