fix MinGW build

* Use the `SIZEOF()` macro in ldscript, to find if a section is empty or not defined.
* Ensure that `-fno-asynchronous-unwind-tables` is used when checking objcopy behaviour, by setting `CPPFLAGS` in configure.ac. Without this, the objcopy absolute address test fails because gcc on MinGW creates a `.eh_frame` section, that objcopy can't handle.
* Note: To get a working grub4dos using MinGW, you may have to downgrade gcc to 4.6.2 using:
```
     mingw-get upgrade gcc=4.6.2-1
     mingw-get install mpc=0.8.1-1
```
  MinGW users should also run autogen.sh or bootstrap.sh before invoking configure.