OS_CYGWIN is used to assume cygpath, Win32-isms

--- boost_1_48_0/tools/build/src/engine/jam.h
+++ boost_1_48_0/tools/build/src/engine/jam.h
@@ -245,7 +245,6 @@
 #endif
 #if defined(__cygwin__) || defined(__CYGWIN__)
     #define OSMINOR "OS=CYGWIN"
-    #define OS_CYGWIN
 #endif
 #if defined(__FreeBSD__) && !defined(__DragonFly__)
     #define OSMINOR "OS=FREEBSD"