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

diff --git a/include/winbase.h b/include/winbase.h
index 95d9782..0aebe0d 100644
--- a/include/winbase.h
+++ b/include/winbase.h
@@ -1233,8 +1233,8 @@
 BOOL        WINAPI EqualSid(PSID, PSID);
 BOOL        WINAPI EqualPrefixSid(PSID,PSID);
 DWORD       WINAPI EraseTape(HANDLE,DWORD,BOOL);
-VOID        WINAPI ExitProcess(DWORD) WINE_NORETURN;
-VOID        WINAPI ExitThread(DWORD) WINE_NORETURN;
+VOID DECLSPEC_NORETURN WINAPI ExitProcess(DWORD);
+VOID DECLSPEC_NORETURN WINAPI ExitThread(DWORD);
 DWORD       WINAPI ExpandEnvironmentStringsA(LPCSTR,LPSTR,DWORD);
 DWORD       WINAPI ExpandEnvironmentStringsW(LPCWSTR,LPWSTR,DWORD);
 #define     ExpandEnvironmentStrings WINELIB_NAME_AW(ExpandEnvironmentStrings)