Use the standard DllMain entry point.

dlls/user/Makefile.in

@@ -7,7 +7,6 @@ MODULE    = user32.dll
 IMPORTS   = gdi32 advapi32 kernel32 ntdll
 ALTNAMES  = user.exe ddeml.dll display.drv keyboard.drv mouse.drv
 EXTRALIBS = $(LIBUNICODE)
-DLLMAIN   = UserClientDllInitialize@12
 
 LDDLLFLAGS = @LDDLLFLAGS@
 SYMBOLFILE = $(MODULE).tmp.o

dlls/user/user32.spec

@@ -599,7 +599,7 @@
 @ stdcall UnregisterHotKey(long long)
 @ stub UpdatePerUserSystemParameters
 @ stdcall UpdateWindow(long)
-@ stdcall UserClientDllInitialize(long long ptr)
+@ stdcall UserClientDllInitialize(long long ptr) DllMain
 # @ stub UserIsSystemResumeAutomatic
 # @ stub UserSetDeviceHoldState
 @ stub UserHandleGrantAccess

dlls/user/user_main.c

@@ -291,9 +291,9 @@ static void thread_detach(void)
 /***********************************************************************
  *           UserClientDllInitialize  (USER32.@)
  *
- * USER dll initialisation routine
+ * USER dll initialisation routine (exported as UserClientDllInitialize for compatibility).
  */
-BOOL WINAPI UserClientDllInitialize( HINSTANCE inst, DWORD reason, LPVOID reserved )
+BOOL WINAPI DllMain( HINSTANCE inst, DWORD reason, LPVOID reserved )
 {
     BOOL ret = TRUE;
     switch(reason)