MDEV-19740: Have MYSQL_MAINTAINER_MODE only enable -Werror

Let us enable all GCC and clang warnings independently of the
MYSQL_MAINTAINER_MODE setting for both Debug and RelWithDebInfo
builds, and have MYSQL_MAINTAINER_MODE only enable -Werror.

The default setting of MYSQL_MAINTAINER_MODE=AUTO will continue
to apply the -Werror only to CMAKE_BUILD_TYPE=Debug. To build
a debug version without -Werror, MYSQL_MAINTAINER_MODE=OFF can
be used.

cmake/maintainer.cmake

 # Copyright (c) 2010, 2014, Oracle and/or its affiliates. All rights reserved.
+# Copyright (c) 2020, MariaDB
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -28,11 +29,16 @@ SET(MY_WARNING_FLAGS
   -Woverloaded-virtual
   -Wvla
   -Wwrite-strings
-  -Werror
   )
 
+FOREACH(F ${MY_WARNING_FLAGS})
+  MY_CHECK_AND_SET_COMPILER_FLAG(${F} DEBUG RELWITHDEBINFO)
+ENDFOREACH()
+
+SET(MY_ERROR_FLAGS -Werror)
+
 IF(CMAKE_COMPILER_IS_GNUCC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "6.0.0")
-  SET(MY_WARNING_FLAGS ${MY_WARNING_FLAGS} -Wno-error=maybe-uninitialized)
+  SET(MY_ERROR_FLAGS ${MY_ERROR_FLAGS} -Wno-error=maybe-uninitialized)
 ENDIF()
 
 IF(MYSQL_MAINTAINER_MODE MATCHES "OFF")
@@ -41,7 +47,7 @@ ELSEIF(MYSQL_MAINTAINER_MODE MATCHES "AUTO")
   SET(WHERE DEBUG)
 ENDIF()
 
-FOREACH(F ${MY_WARNING_FLAGS})
+FOREACH(F ${MY_ERROR_FLAGS})
   MY_CHECK_AND_SET_COMPILER_FLAG(${F} ${WHERE})
 ENDFOREACH()