Fix bug #18995 with compiling w32heap.c with -funroll-loops.

 src/w32heap.c (allocate_heap): Set the lower limit of heap at 8MB.

src/ChangeLog

+2014-11-09  Eli Zaretskii  <eliz@gnu.org>
+
+	* w32heap.c (allocate_heap): Set the lower limit of heap at 8MB.
+	(Bug#18995)
+
 2014-11-09  Jan Djärv  <jan.h.d@swipnet.se>
 
 	* nsterm.h (EmacsScroller): judge returns bool.

src/w32heap.c

@@ -96,7 +96,7 @@ static char *
 allocate_heap (void)
 {
 #ifdef _WIN64
-  size_t size = 0x4000000000ull; /* start by asking for 32GB */
+  size_t size = 0x4000000000ull; /* start by asking for 256GB */
 #else
   /* We used to start with 2GB here, but on Windows 7 that would leave
      too little room in the address space for threads started by
@@ -106,7 +106,7 @@ allocate_heap (void)
 #endif
   void *ptr = NULL;
 
-  while (!ptr && size > 0x00100000)
+  while (!ptr && size >= 0x00800000)
     {
       reserved_heap_size = size;
       ptr = VirtualAlloc (NULL,