Recommendations When Using gcc

It is recommended to use -O3 -mtune=native to achieve maximum speed during LightGBM training.

Using Intel Ivy Bridge CPU on 1M x 1K Bosch dataset, the performance increases as follow:

Compilation Flag	Performance Index
`-O2 -mtune=core2`	100.00%
`-O2 -mtune=native`	100.90%
`-O3 -mtune=native`	102.78%
`-O3 -ffast-math -mtune=native`	100.64%

You can find more details on the experimentation below:

Some explanatory pictures:

./_static/images/gcc-table.png

./_static/images/gcc-bars.png

./_static/images/gcc-chart.png

./_static/images/gcc-comparison-1.png

./_static/images/gcc-comparison-2.png

./_static/images/gcc-meetup-1.png

./_static/images/gcc-meetup-2.png