use uname instead of often outdated header file

auto/os/linux

diff --git a/auto/os/linux b/auto/os/linux
index 62bc625..3c2c841 100644
--- a/auto/os/linux
+++ b/auto/os/linux
@@ -16,8 +16,8 @@
 
 # Linux kernel version
 
-version=`grep "#define LINUX_VERSION_CODE" /usr/include/linux/version.h \
-         | sed -e 's/^.* \(.*\)$/\1/'`
+version=$((`uname -r \
+         | sed 's/^\([^.]*\)\.\([^.]*\)\.\([^.-]*\).*/\1*256*256+\2*256+\3/'`))
 
 version=${version:-0}