Set Off_t et al correctly for USE_LARGE_FILES builds on Windows

The canned config files used to build miniperl.exe (with some updates
applied by Makefile / makefile.mk) have the non-USE_LARGE_FILES settings
for Off_t, LSEEKSIZE and Off_t_size, resulting in an uninitialized memory
read in win32_fseek() in miniperl.exe, which recently started causing
exetype.pl to write a 4GB wperl.exe during the build of a 64-bit perl.

Change Makefile / makefile.mk to use the correct settings for the build of
miniperl.exe. This fixes perl #121471.
Fix DllMain definition to fix building with gcc-4.8.1 from www.mingw.org

This partly fixes perl #121643.
Remove unused variable