configure.ac: replace -O20 optimization flags with -O2.
configure.ac: replace -O20 optimization flags with -O2.

The idea was to trigger maximum optimizations, but to date
compilers only go up to -O3, so this in ambiguous.

Signed-off-by: Ralph Giles <65585f72bf62a11d2f4105aa30c20fe94efea46f@thaumas.net>

ShiftMediaProject/ogg,ShiftMediaProject/ogg,xiph/ogg,xiph/ogg