klibc.bbclass: shared include for building against klibc

* from org.openembedded.dev
* reworked in meta-zaurus
* added missing quotes
* explain more clearly why the compiler/linker flags are reset

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>