Implement aligned_alloc().

Implement aligned_alloc(), which was added in the C11 standard.  The
function is weakly specified to the point that a minimally compliant
implementation would be painful to use (size must be an integral
multiple of alignment!), which in practice makes posix_memalign() a
safer choice.
diff --git a/Makefile.in b/Makefile.in
index 6286455..01ed083 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -63,9 +63,9 @@
 DOCS_HTML := $(DOCS_XML:@objroot@%.xml=@srcroot@%.html)
 DOCS_MAN3 := $(DOCS_XML:@objroot@%.xml=@srcroot@%.3)
 DOCS := $(DOCS_HTML) $(DOCS_MAN3)
-CTESTS := @srcroot@test/allocated.c @srcroot@test/bitmap.c \
-	@srcroot@test/mremap.c @srcroot@test/posix_memalign.c \
-	@srcroot@test/thread_arena.c
+CTESTS := @srcroot@test/aligned_alloc.c @srcroot@test/allocated.c \
+	@srcroot@test/bitmap.c @srcroot@test/mremap.c \
+	@srcroot@test/posix_memalign.c @srcroot@test/thread_arena.c
 ifeq (@enable_experimental@, 1)
 CTESTS += @srcroot@test/allocm.c @srcroot@test/rallocm.c
 endif