While trying to compile gdc 4.8.2 on Cygwin following the generic install
instrcutions at http://wiki.dlang.org/GDC/Installation/Generic I encountered
the following errors.  The gcc source I used is the one provided when you tell
cygwin's setup.exe to give you the source for it's gcc-core package version
4.8.2-2

../../gcc-4.8.2/gcc/d/dfrontend/filename.c: In static member function ‘static
int FileName::ensurePathExists(const char*)’:
../../gcc-4.8.2/gcc/d/dfrontend/filename.c:597:43: error: ‘sep’ was not
declared in this scope
             if (path[strlen(path) - 1] != sep)
                                           ^
../../gcc-4.8.2/gcc/d/dfrontend/filename.c:606:21: error: ‘r’ was not declared
in this scope
                 if (r)
                     ^
../../gcc-4.8.2/gcc/d/dfrontend/filename.c:611:25: error: ‘errno’ was not
declared in this scope
                     if (errno != EEXIST)
                         ^
../../gcc-4.8.2/gcc/d/dfrontend/filename.c:611:34: error: ‘EEXIST’ was not
declared in this scope
                     if (errno != EEXIST)
                                  ^
When looking at the source file there appears to be an impossible #if condition
of:
#if _WIN32
...
#elif POSIX
...
#endif

I am no expert when it comes system related compiling but I would have thought
that these should just be #else statements or perhaps an some #if including
__CYGWIN__