[Box Backup] Latest Box Backup trunk R2547 fails to build on MingW

[Achim]

For Box Backup R2547 after following the instructions at 
<http://www.boxbackup.org/trac/wiki/CompileWithMinGW>

[...]
   [CXX]     BackupStoreRefCountDatabase.cpp
BackupStoreRefCountDatabase.cpp: In static member function `static void 
BackupStoreRefCountDatabase::Create(const 
BackupStoreAccountDatabase::Entry&, bool)':
BackupStoreRefCountDatabase.cpp:91: error: `FileExists' undeclared 
(first use this function)
BackupStoreRefCountDatabase.cpp:91: error: (Each undeclared identifier 
is reported only once for each function it appears in.)
make[4]: *** 
[../../release/lib/backupstore/BackupStoreRefCountDatabase.o] Error 1
make[4]: Leaving directory `/usr/src/boxi.645/boxbackup/lib/backupstore'
make[3]: *** [dep_modules] Error 2
make[3]: Leaving directory `/usr/src/boxi.645/boxbackup/bin/bbstored'
make[2]: *** [release/bin/bbstored/bbstored.exe] Error 2
make[2]: Leaving directory `/usr/src/boxi.645/boxbackup'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/usr/src/boxi.645'
make: *** [all] Error 2