fix compilation with ASAN

if the property is not found, set it to the empty string,
otherwise it'll show as libmysql_link_flags-NOTFOUND on the linker
command line, and the linker won't like it.

Also, don't specify LINK_FLAG_NO_UNDEFINED twice, MERGE_LIBRARIES
already put it into LINK_FLAGS.

libmysql/CMakeLists.txt

@@ -466,10 +466,13 @@ IF(NOT DISABLE_SHARED)
       OUTPUT_NAME mysqlclient 
       VERSION "${OS_SHARED_LIB_VERSION}" 
       SOVERSION "${SHARED_LIB_MAJOR_VERSION}")
-    IF(LINK_FLAG_NO_UNDEFINED OR VERSION_SCRIPT_LINK_FLAGS)
+    IF(VERSION_SCRIPT_LINK_FLAGS)
       GET_TARGET_PROPERTY(libmysql_link_flags libmysql LINK_FLAGS)
+      IF(NOT libmysql_link_flags)
+        SET(libmysql_link_flags)
+      ENDIF()
       SET_TARGET_PROPERTIES(libmysql PROPERTIES LINK_FLAGS 
-        "${libmysql_link_flags} ${LINK_FLAG_NO_UNDEFINED} ${VERSION_SCRIPT_LINK_FLAGS}")
+        "${libmysql_link_flags} ${VERSION_SCRIPT_LINK_FLAGS}")
     ENDIF() 
     # clean direct output needs to be set several targets have the same name
     #(mysqlclient in this case)