Add OPENSSL_API_3_0 define to documentation

docs/Building.md

@@ -179,6 +179,8 @@ make build COPT="-DNDEBUG -DNO_CGI"
 | `SSL_ALREADY_INITIALIZED`    | do not initialize libcrypto                                         |
 | `OPENSSL_API_1_0`            | Use OpenSSL V1.0.x interface                                        |
 | `OPENSSL_API_1_1`            | Use OpenSSL V1.1.x interface                                        |
+| `OPENSSL_API_3_0`            | Use OpenSSL V3.x interface                                          |
+| `USE_MBEDTLS`                | Use MbedTLS (cannot be combined with OPENSSL_API_*)                 |
 |                              |                                                                     |
 | `BUILD_DATE`                 | define as a string to be used as build id instead of __DATE__       |
 |                              |                                                                     |

src/civetweb.c

@@ -1556,10 +1556,15 @@ static int mg_openssl_initialized = 0;
 #endif
 #if !defined(OPENSSL_API_1_0) && !defined(OPENSSL_API_1_1)                     \
     && !defined(OPENSSL_API_3_0) && !defined(USE_MBEDTLS)
-#error "Please define OPENSSL_API_1_0 or OPENSSL_API_1_1"
+#error "Please define OPENSSL_API_#_# or USE_MBEDTLS"
 #endif
 #if defined(OPENSSL_API_1_0) && defined(OPENSSL_API_1_1)                       \
-    && defined(OPENSSL_API_3_0)
+#error "Multiple OPENSSL_API versions defined"
+#endif
+#if defined(OPENSSL_API_1_1) && defined(OPENSSL_API_3_0)                       \
+#error "Multiple OPENSSL_API versions defined"
+#endif
+#if defined(OPENSSL_API_1_0) && defined(OPENSSL_API_3_0)                       \
 #error "Multiple OPENSSL_API versions defined"
 #endif
 #if (defined(OPENSSL_API_1_0) || defined(OPENSSL_API_1_1)                      \