OS_* defines not always set correctly

[chausner]

logging.h.in depends on OS_* defines:

glog/src/glog/logging.h.in

Line 104 in 8f9ccfe

#elif defined(OS_WINDOWS)

. These defines are set in utilities.h:

glog/src/utilities.h

Line 37 in 2412db3

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32__)

utilities.h includes logging.h but not the other way round. Thus, if someone includes logging.h without having included utilities.h first, these defines are not set, and compilation will fail on e.g. Windows because it tries to compile gcc-specific code.

A fix could be to move out the OS_* defines into a separate header file which could get included by both logging.h.in and utilities.h.