Fixes

Hi @count0 more modernisation cleanups. A few notes:

In general, if a user specifies, say CXXFLAGS="-g", you shouldn't override his choice, i.e., you shouldn't force -O3 on them. I've used the -O3-by-default pattern now:

1. ./configure without CXXFLAGS gives you -O3
2. ./configure with CXXFLAGS=foo does not add -O3 to the command line.

AC_CANONICAL_TARGET is a common misunderstanding. AC_CANONICAL_TARGET is not the target you're compiling for. It's the platform the resulting binary will be generating code for, i.e., it's only relevant for code-generating tools. AC_CANONICAL_HOST is what you want in 99% of cases.