Use new .dll names on Windows x64 for OpenSSL 1.1.x

In OpenSSL 1.1.x, the following .dlls have new names:
    libeay64.dll -> libcrypto-1_1-x64.dll
    ssleay64.dll -> libssl-1_1-x64.dll

While work was done in https://github.com/civetweb/civetweb/issues/370
(commit 0733fae) to have civetweb use OpenSSL 1.1.x's new API, the
Windows .dll names were not changed. This causes civetweb to throw an
error since it can't find libeay or ssleay. A workaround would be to
copy the new dlls and rename them as the old .dlls. With this commit,
such a workaround is no longer necessary if civetweb is build with the
OPENSSL_API_1_1 flag.

See https://mta.openssl.org/pipermail/openssl-users/2017-June/005977.html
for more details about the relevant OpenSSL change.

src/civetweb.c

@@ -569,19 +569,27 @@ typedef long off_t;
 
 #if defined(_WIN64) || defined(__MINGW64__)
 #if !defined(SSL_LIB)
+#if defined(OPENSSL_API_1_1)
+#define SSL_LIB "libssl-1_1-x64.dll"
+#else /* OPENSSL_API_1_1 */
 #define SSL_LIB "ssleay64.dll"
-#endif
+#endif /* OPENSSL_API_1_1 */
+#endif /* SSL_LIB */
 #if !defined(CRYPTO_LIB)
+#if defined(OPENSSL_API_1_1)
+#define CRYPTO_LIB "libcrypto-1_1-x64.dll"
+#else /* OPENSSL_API_1_1 */
 #define CRYPTO_LIB "libeay64.dll"
-#endif
-#else
+#endif /* OPENSSL_API_1_1 */
+#endif /* CRYPTO_LIB */
+#else /* defined(_WIN64) || defined(__MINGW64__) */
 #if !defined(SSL_LIB)
 #define SSL_LIB "ssleay32.dll"
-#endif
+#endif /* SSL_LIB */
 #if !defined(CRYPTO_LIB)
 #define CRYPTO_LIB "libeay32.dll"
-#endif
-#endif
+#endif /* CRYPTO_LIB */
+#endif /* defined(_WIN64) || defined(__MINGW64__) */
 
 #define O_NONBLOCK (0)
 #if !defined(W_OK)