README: Add a minimum GLM versions.

0.9.2.7 is what is currently used in olddeps build tests. Even Ubuntu
doesn't ship with any older versions.
It might actually work with an older version (0.9.2.0 doesn't) but meh.
README: Add a minimum Freetype version

Again, might actually work with older versions, but no-one ships even
2.3.0 anymore.
Drop support for Boost < 1.48

Everyone except Ubuntu lucid and Debian squeeze has (or should have!)
newer versions already and those two don't have GLM anyway.

Removing support for older Boost versions allows us to get rid of some
compatibility hacks.
Define BOOST_ALL_NO_LIB globally

Instead of a few BOOST_DATE_TIME_NO_LIB defines before #includes.
Drop support for Qt < 4.7
Merge arx_assert_msg() into arx_assert()