MSVC: fix compile errors due to macro redefinitions
MSVC: fix compile errors due to macro redefinitions

Skip errno.h definitions if they are already defined.

Signed-off-by: Karsten Blees <955b63282c975ba379c4995cac1c2e1e7dfebe62@dcon.de>
Acked-by: Sebastian Schuberth <77397652b38ec60b50e56236b813774ce842bcdf@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git