manual: Bump required gcc version to 4.1.

manual: Bump required gcc version to 4.1.

gcc-3 is not good enough any more since it has no support for weak
references, which were introduced half a year ago in commit 9bf6dc2e
(error.h: Never call (para_)strerror() on osl errors).

gcc-3.3 was released in 2003, gcc-4.2 in 2007. It should be OK to
require gcc-4.2 from now on.