This patch makes the exeception specifier of libiberty's 'asprintf'
and 'vasprintf' declarations match those of glibc to work around the
problem described at <https://gcc.gnu.org/ml/gcc-help/2016-04/msg00039.html>.

The problem in part stems from the fact that libiberty is configured
without _GNU_SOURCE (thus, it sets HAVE_DECL_ASPRINTF to 0), whereas libcc1
is configured and built with _GNU_SOURCE, hence the conflicting declarations.

--- gcc-5.3.0/include/libiberty.h	2016-04-23 22:45:46.262709079 +0200
+++ gcc-5.3.0/include/libiberty.h	2016-04-23 22:45:37.110635439 +0200
@@ -625,7 +625,7 @@ extern int pwait (int, int *, int);
 /* Like sprintf but provides a pointer to malloc'd storage, which must
    be freed by the caller.  */
 
-extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;
+extern int asprintf (char **, const char *, ...) __THROWNL ATTRIBUTE_PRINTF_2;
 #endif
 
 /* Like asprintf but allocates memory without fail. This works like
@@ -637,7 +637,7 @@ extern char *xasprintf (const char *, ..
 /* Like vsprintf but provides a pointer to malloc'd storage, which
    must be freed by the caller.  */
 
-extern int vasprintf (char **, const char *, va_list) ATTRIBUTE_PRINTF(2,0);
+extern int vasprintf (char **, const char *, va_list) __THROWNL ATTRIBUTE_PRINTF(2,0);
 #endif
 
 /* Like vasprintf but allocates memory without fail. This works like