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Description of problem:
I failed to compile 1) while I could compile 2) successfully. 

1)
---test1.c----
#define _XOPEN_SOURCE
#define _XOPEN_SOURCE_EXTENDED
#include<ucontext.h>
main() {}
--------------

# gcc test1.c
In file included from /usr/include/ucontext.h:28,
                 from test.c:6:
/usr/include/sys/ucontext.h:46: field `_mc' has incomplete type
/usr/include/sys/ucontext.h:49: dereferencing pointer to incomplete type
/usr/include/sys/ucontext.h:49: size of array `_pad' has non-integer type
gcc: Internal compiler error: program cc1 got fatal signal 11


2)
---test2.c----
#define _BSD_SOURCE
#include<ucontext.h>
main() {}
--------------

I think the cause is in /usr/include/signal.h.
In signal.h, if __USE_XOPEN_EXTENDED was defined the valuable of 
mcontext_t type is used but the type is not defined without __USE_BSD.
 
I suppose /usr/include/signal.h should be applied a patch like the 
following.

--- signal.h    Tue May 22 11:46:26 2001
+++ /usr/include/signal.h.modified      Tue May 22 11:47:35 2001
@@ -311,6 +311,9 @@

 #endif /*  use BSD.  */

+#if !defined __USE_BSD && defined __USE_XOPEN_EXTENDED
+# include <bits/sigcontext.h>
+#endif

 #if defined __USE_BSD || defined __USE_XOPEN_EXTENDED


How reproducible:
Always

Steps to Reproduce:
1.gcc -c test1.c
2.gcc -c test2.c
3.
	

Additional info: