Actually use of /usr/src/linux is *highly* discouraged. It is far better to use the /usr/src/linux-... or better still create a /usr/src/local/linux or some such directory and use that for compiling kernels and tweaking and experimenting. You can royally jack up your install by using /usr/src/linux as a kernel playground.
Unless Linus got tired of people jacking up their systems and changed the behavior of the kernel libraries.
-----Original Message----- From: Frank Wiles
... /usr/src/linux isn't a "magic place" or anything.