Convert cord tests to valid C++ code

Convert cord tests to valid C++ code

Issue #201 (bdwgc).

* cord/tests/de.c (main): Cast buf to char* when passed to setvbuf().
* cord/tests/de_win.c (de_error): Change "char*" to "const char*" for
the argument (because ISO C++ forbids converting a string constant to
char*).
* cord/tests/de_win.h (de_error): Likewise.
* cord/tests/de_win.c (WinMain): Change type of hAccel local variable
from HANDLE to HACCEL; cast GetStockObject(WHITE_BRUSH) result to
HBRUSH type.
* cord/tests/de_win.c (plain_chars, control_chars): Cast
GC_MALLOC_ATOMIC() result to char*.
* cord/tests/de_win.c (WndProc): Change type of hInstance static
variable from HANDLE to HINSTANCE.