library: eliminate inappropriate '__BEGIN_DECLS' macro

This patch simply eliminates that glibc specific macro
from all header files which contain no public callable
functions. After all, if user code can't link to them,
then protection from C++ name mangling is unnecessary.

[ we also remove any related '#include <features.h>' ]

Signed-off-by: Jim Warner <james.warner@comcast.net>

include/signals.h

@@ -11,8 +11,6 @@
  * GNU Library General Public License for more details.
  */
 
-__BEGIN_DECLS
-
 extern const int number_of_signals;
 
 extern const char *get_sigtable_name(int row);
@@ -32,5 +30,4 @@ extern void pretty_print_signals(void);
 
 extern void unix_print_signals(void);
 
-__END_DECLS
 #endif