configure.ac: backport [24902] use build_os instead of SYS for detecting...

configure.ac: backport [24902] use build_os instead of SYS for detecting cygwin for hostname check because when we deal with mingw32 compiler (mno-cygwin mode) then SYS value isn't cygwin but (forced to) mingw32

configure.ac

@@ -5468,7 +5468,7 @@ AC_SUBST(VERSION_MAJOR)
 AC_SUBST(VERSION_MINOR)
 AC_SUBST(VERSION_REVISION)
 AC_DEFINE_UNQUOTED(VLC_COMPILE_BY, "`whoami`", [user who ran configure]) 
-if test "${SYS}" = "cygwin"
+if test "${build_os}" = "cygwin"
 then
     AC_DEFINE_UNQUOTED(VLC_COMPILE_HOST, "`hostname`", [host which ran configure])
 else