[v3] doxygen updates

[Benjamin Kosnik]

Changes to support doxygen output for trunk and gcc-4_3-branch.
Generated output should be as on the gcc website. 

Notable changes include:
1) use doxygen default css style
2) add markup for some tr1 and C++0x bits missing (unordered_*,
type_traits, others)
3) simplified brief comments
4) downgrade to doxygen 1.5.1. There were issues with newer doxygen and
filenames, nested namespaces, etc.

This gives the generated docs for 4.3/trunk equivalent output as
for 4.2. I will wait a bit for the 4_3-branch check in. 

gcc
gcc-4_3-branch

tested x86_64/linux

-benjamin