Build by default with optimization level -O2 for GCC

before -O0 was used by default, that caused pretty bad performance.

Amd64, i6700k, 4200MHz, GNU/Linux (non-RT kernel), gcc 7.2.0

-------------------------------------
Optimization | EN/ENO |no EN/ENO |
-------------------------------------
default | 11 | 9.5 |
-O3 | 3.9 | 5.2 |
-O2 | 4 | 4.8 |
-Os | 4.1 | 3.5 |
-Ofast | 3.9 | 5.2 |
-------------------------------------

ARM, BBB Cortex-A8, 600Mhz, GNU/Linux, gcc 4.6.3

-------------------------------------
Optimization | EN/ENO |no EN/ENO |
-------------------------------------
default | 273 | 226 |
-O3 | 141.8 | 106.2 |
-O2 | 142 | 107 |
-Os | 152.5 | 112.2 |
-Ofast | 141.7 | 106.2 |
-------------------------------------

For embedded systems with size constaints (like Cortex-Mx, AVR and so
on) I usually use -Os. It gets pretty good results. For
GNU/Linux-based systems -O2 is usually a good choice, as you see the
test results.

     1

<?xml version="1.0"?>

     2

<purchaseOrder orderDate="1999-10-20">

     3

  <shipTo country="US">

     4

    <name>Alice Smith</name>

     5

    <street>123 Maple Street</street>

     6

    <city>Mill Valley</city>

     7

    <state>CA</state>

     8

    <zip>90952</zip>

     9

  </shipTo>

    10

  <billTo country="US">

    11

    <name>Robert Smith</name>

    12

    <street>8 Oak Avenue</street>

    13

    <city>Old Town</city>

    14

    <state>PA</state>

    15

    <zip>95819</zip>

    16

  </billTo>

    17

  <comment>Hurry, my lawn is going wild!</comment>

    18

  <items>

    19

    <item partNum="872-AA">

    20

      <productName>Lawnmower</productName>

    21

      <quantity>1</quantity>

    22

      <USPrice>148.95</USPrice>

    23

      <comment>Confirm this is electric</comment>

    24

    </item>

    25

    <item partNum="926-AA">

    26

      <productName>Baby Monitor</productName>

    27

      <quantity>1</quantity>

    28

      <USPrice>39.98</USPrice>

    29

      <shipDate>1999-05-21</shipDate>

    30

    </item>

    31

  </items>

    32

</purchaseOrder>