bind/placeholders.hpp workaround for multiply defined anon namespace broken in gcc 4..1

[joequant@…]

in bind/placeholders.hpp there is a workaround for compiled headers sharing anonymous name space information causing multiply defined errors with placeholders

see ​http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29085

Unfortunately the ifdef for activating the workaround in boost/bind/placeholders.hpp is incorrect as the bug was fixed in gcc 4.2 and still exists in gcc 4.1

The check

defined(GNUC) && (GNUC * 100 + GNUC_MINOR == 400)

should be replaced with

defined(GNUC) && (GNUC * 100 + GNUC_MINOR < 420)

[Steven Watanabe]

Duplicates #4172