configure.ac: Fix have mbedtls lib, but no mbedtls-dev issue

Do not make it a hard error if mbedtls library exists, but the mbedtls
development environment is missing.

Just remove mbedtls support if this is the case.

configure.ac

@@ -415,7 +415,8 @@ if test "x$build_dtls" = "xyes"; then
                        return 0;
                      }]])],
                [mbedtls_version=$(./conftest$EXEEXT)],
-               [AC_MSG_ERROR(Failed to determine Mbed TLS version)])
+               [AC_MSG_WARN(Failed to determine Mbed TLS version)
+                have_mbedtls=no])
             LIBS=$local_MbedTLS_save_LIBS
             AC_LANG_POP(C)
         fi