Added a GNU C implementation of some of the DECLSPEC_ macros and used
them wherever possible.

diff --git a/scheduler/sysdeps.c b/scheduler/sysdeps.c
index 5101dd3..df3a870 100644
--- a/scheduler/sysdeps.c
+++ b/scheduler/sysdeps.c
@@ -233,7 +233,7 @@
 /***********************************************************************
  *           SYSDEPS_CallOnStack
  */
-void SYSDEPS_CallOnStack( void (*func)(LPVOID), LPVOID arg ) WINE_NORETURN;
+void DECLSPEC_NORETURN SYSDEPS_CallOnStack( void (*func)(LPVOID), LPVOID arg );
 #ifdef __i386__
 #ifdef __GNUC__
 __ASM_GLOBAL_FUNC( SYSDEPS_CallOnStack,