configure: experimental cleanup of GCC optmisation flags
configure: experimental cleanup of GCC optmisation flags

Cleanup GCC-based optimisation flags.  Use -O3 instead of our -O6
-fexpensive ..., since -O3 already includes all of those flags, except
-funroll-all-loops.  GCC knows best about the target platform it
compiles for, hence it also knows the best argument for -falign-loops
(malign-loops doesn't exist, so it wasn't in effect anyway).

On Darwin, use the special -fast flag (GCC 4.6's equivalent -Ofast???)
to enable ultimate optimisation flags, though they might break the
resulting binary.

This commit is experimental in the sense that it is meant to be compared
to the last run, such that we can see differences in
- compilation speed
- running speed
- test output

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb