Bug 979913: Conditionally define PAGE_SIZE, r=khuey

With this patch the PAGE_SIZE value from the system is used if
possible. The code still assumes that the PAGE_SIZE is 4096, but
emits a warning if the actual value differs. If page size is not
defined by the system, it is set to 4096 bytes and a warning is
emitted as well.

mozglue/build/Nuwa.cpp

@@ -142,7 +142,12 @@ TLSInfoList;
  * methods or do large allocations on the stack to avoid stack overflow.
  */
 #ifndef NUWA_STACK_SIZE
-#define PAGE_SIZE 4096
+#if !defined(PAGE_SIZE)
+#warning "PAGE_SIZE not defined, using 4096 bytes"
+#define PAGE_SIZE 4096ul
+#elif PAGE_SIZE != 4096ul
+#warning "System page size not 4096 bytes"
+#endif
 #define PAGE_ALIGN_MASK 0xfffff000
 #define NUWA_STACK_SIZE (1024 * 128)
 #endif