Two issues about the cygwin build

[Harald Maier]

Hello all,

after the today checkout I did a build in a cygwin environment. The
following two issues were raised:

1) sheap.c: The size of the heap (7 * 1024 *1024) is no longer
enough for bootstrapping. After increasing the heap size to (8 * 1024 *
1024) bootstrapping did work.

2) alloc.c: The function proto type of 

  void refill_memory_reserve ();

is inside of GC_MALLOC_CHECK. Under cygwin this code is not valid so
the compiler did not find that prototype. After I moved the prototype
outside of GC_MALLOC_CHECK the file compiled fine.

Harald