MYSQL_MAINTAINER_MODE fixes

* MSVC check was lost in a bad merge
* add a comment
* move AIX check where it belongs and where it would still
  allow -Werror, if desired

cmake/maintainer.cmake

@@ -14,7 +14,8 @@
 # along with this program; if not, write to the Free Software
 # Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335  USA
 
-IF(MYSQL_MAINTAINER_MODE STREQUAL "NO")
+IF(MSVC OR MYSQL_MAINTAINER_MODE STREQUAL "NO")
+  # Windows support is in cmake/os/Windows.cmake, not here
   RETURN()
 ENDIF()
 
@@ -40,11 +41,7 @@ FOREACH(F ${MY_WARNING_FLAGS})
   MY_CHECK_AND_SET_COMPILER_FLAG(${F} DEBUG RELWITHDEBINFO)
 ENDFOREACH()
 
-IF(CMAKE_SYSTEM_NAME MATCHES AIX)
-  SET(MY_ERROR_FLAGS "")
-ELSE()
-  SET(MY_ERROR_FLAGS -Werror)
-ENDIF()
+SET(MY_ERROR_FLAGS -Werror)
 
 IF(CMAKE_COMPILER_IS_GNUCC AND CMAKE_C_COMPILER_VERSION VERSION_LESS "6.0.0")
   SET(MY_ERROR_FLAGS ${MY_ERROR_FLAGS} -Wno-error=maybe-uninitialized)

cmake/os/AIX.cmake

@@ -33,3 +33,6 @@ ELSE()
   SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGE_FILES -maix64 -pthread -mcmodel=large")
   SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGE_FILES -maix64 -pthread -mcmodel=large")
 ENDIF()
+
+# make it WARN by default, not AUTO (that implies -Werror)
+SET(MYSQL_MAINTAINER_MODE "WARN" CACHE STRING "Enable MariaDB maintainer-specific warnings. One of: NO (warnings are disabled) WARN (warnings are enabled) ERR (warnings are errors) AUTO (warnings are errors in Debug only)")