kbuild: Fix compiling/installing as different users

"make bzImage && sudo make install" had the problem that during
the "sudo make install" the build system would notice that the information
in include/linux/compile.h is not accurate (it says "compiled by <user>",
but we are root), thus causing compile.h to be updated and leading to
some recompiles.

We now only update "compile.h" if the current user is the owner of
include/linux/autoconf.h, i.e. the user who did the "make *config". So the
above sequence will correctly state "compiled by <user>".

scripts/mkcompile_h

@@ -3,6 +3,17 @@ ARCH=$2
 SMP=$3
 CC=$4
 
+# If compile.h exists already and we don't own autoconf.h
+# (i.e. we're not the same user who did make *config), don't
+# modify compile.h
+# So "sudo make install" won't change the "compiled by <user>"
+# do "compiled by root"
+
+if [ -r $TARGET -a ! -O ../include/linux/autoconf.h ]; then
+  echo ' (not modified)'
+  exit 0
+fi
+
 if [ -r ../.version ]; then
   VERSION=`cat ../.version`
 else