MDEV-9298 : Build failure when linking libmysql.

If GCC or CLang compile with link time optimization (-flto),
they throw an error during link , when lto sees a
function (e.g mysql_real_connect) is redeclared as "external void *"
in libmysql_exports.cc

The fix disables -flto for generated libmysql_exports.cc

cmake/libutils.cmake

@@ -87,6 +87,11 @@ MACRO(CREATE_EXPORT_FILE VAR TARGET API_FUNCTIONS)
     ENDFOREACH()
     SET(CONTENT "${CONTENT} (void *)0\n}\;")
     CONFIGURE_FILE_CONTENT(${CONTENT} ${EXPORTS})
+    # Avoid "function redeclared as variable" error
+    # when using gcc/clang option -flto(link time optimization)
+    IF(" ${CMAKE_C_FLAGS} ${CMAKE_CXX_FLAGS} " MATCHES " -flto")
+      SET_SOURCE_FILES_PROPERTIES(${EXPORTS} PROPERTIES COMPILE_FLAGS "-fno-lto")
+    ENDIF()
     SET(${VAR} ${EXPORTS})
   ENDIF()
 ENDMACRO()