[Bug bootstrap/58918] [4.9 regression] cilk #includes alloc.h unconditionally, even when not present

[joseph at codesourcery dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58918

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery dot 
com> ---
XALLOCA is a libiberty macro, target libraries shouldn't be using 
libiberty headers.