When try to emerge glibc-2.11 from older version you might get this error:
checking for cpuid.h... no
configure: error: gcc must provide the
* ERROR: sys-libs/glibc-2.11.2 failed:
* failed to configure glibc
To solve it get the cpuid.h file from official GCC respositories and put it into /usr/include. Check the name of the downloaded file to be “cpuid.h”.
Now, glibc should compile without problem.