I modified the undefined frexpl in cygwin of make

Fix #2739 stack_extend in mrb_f_send

mrb_f_send needs stack_extend like OP_SEND
Signed-off-by: Go Saito <gos@iij.ad.jp>

Use ptrdiff_t to suppress signedness warning

3df32161 says so but there is no warning
with GCC 4.9 on my Debian GNU/Linux environment.

delete prototypes of undefined functions

Fix a crash bug on raising after realloc

The following program reproduces this problem:

    #include <mruby.h>

    static mrb_value
    recursive(mrb_state *mrb, mrb_value self)
    {
      mrb_int n;

      mrb_get_args(mrb, "i", &n);

      if (n == 0) {
        mrb_raise(mrb, E_RUNTIME_ERROR, "XXX");
      } else {
        mrb_funcall(mrb, self, "recursive", 1, mrb_fixnum_value(n - 1));
      }

      return self;
    }

    int
    main(void)
    {
      mrb_state *mrb;

      mrb = mrb_open();

      mrb_define_method(mrb, mrb->kernel_module, "recursive", recursive,
                        MRB_ARGS_REQ(1));
      mrb_funcall(mrb, mrb_top_self(mrb), "recursive", 1, mrb_fixnum_value(30));

      mrb_close(mrb);
    }

Recursive method call isn't required. It's just for expanding call info
stack.

If mrb_realloc() is called in cipush(), cibase address is changed. So,
we shouldn't compare ci before mrb_realloc() and cibase after
mrb_realloc(). It accesses unknown address and causes crash.

Fix a bug that no expression case doesn't return valid value

Here is a script that reproduces this problem:

     x = case
         when true; 1
         end
     p x # => main # 1 is expected

Fix a bug that if and no return value case can't return true clause value

fix pointer dereference after realloc

Here is a script that reproduce this problem:

     x = if true
           1
         else
           case 2
           when 3
           end
           4
         end
     p x # => nil # 1 is expected

In src/vm.c: mrb_funcall_with_block
stack_extend may realloc mrb->c->stbase, if argv points on mruby's stack,
then it points invalid address after stack_extend.

e.g. src/class.c: mrb_instance_new

This code:

```ruby
class A
  def initialize(a0,a1,a2,a3,a4)
    a0.is_a? Array
  end
end

def f(a0,a1,a2,a3,a4)
  A.new(a0,a1,a2,a3,a4)
  f(a0,a1,a2,a3,a4)
end

f(0,1,2,3,4)
```

is expected to get exception
```
stack level too deep. (limit=(0x40000 - 128)) (SystemStackError)
```

but get segfault.
Signed-off-by: Go Saito <gos@iij.ad.jp>

re-implement mrb_float_to_str()

The new implementation is backwards incompatible, but I couldn't find
any usage outside mruby and I also couldn't think of a different and
good name.

All ISO C99 printf conversion specifiers for floating point numbers and
an optional precision are supported.

It is largely based on code from the MIT licensed musl libc
(http://www.musl-libc.org/) and its floating point printing is exact
(unlike the current code behind Float#to_s).

Make backtraces work again under DISABLE_STDIO

Add more debug flags for GCC based compilers

We can use `#define`-ed macros in GDB with `-g3` flag.

We can run code step by step in GDB with `-O0` flag.

Delete temp variable

mruby-config supports MRUBY_LDFLAGS_BEFORE_LIBS in libmruby.flags.mak

Fix possible segfault when accessing backtrace with MRB_WORD_BOXING.

Fix ensure with yield context on break and return

How to reproduce:

    class A
      def x
        yield
      ensure
        y
      end

      def y
      end
    end

    # Work
    A.new.x do
    end

    # Not work
    # trace:
    # 	[2] /tmp/a.rb:5:in A.x
    # 	[0] /tmp/a.rb:15
    # /tmp/a.rb:5: undefined method 'y' for main (NoMethodError)
    A.new.x do
      break
    end

    # trace:
    # 	[2] /tmp/a.rb:5:in A.call
    # 	[0] /tmp/a.rb:19
    # /tmp/a.rb:5: undefined method 'y' for main (NoMethodError)
    lambda do
      A.new.x do
        return
      end
    end.call

`self` in ensure is broken when yield and break/return are used.

Replace int with mrb_bool in dump_bigendian_p func