Add warning_level directives to build.
Add warning_level directives to build.

No functional change, but shows where this setting can be altered.

twodee/blockly,jstnhuang/blockly,rachel-fenichel/blockly,mark-friedman/blockly,rachel-fenichel/blockly,twodee/blockly,mark-friedman/blockly,rachel-fenichel/blockly,picklesrus/blockly,trinketapp/blockly,nickmain/blockly,nickmain/blockly,rachel-fenichel/blockly,mark-friedman/blockly,rachel-fenichel/blockly,mark-friedman/blockly,google/blockly,nickmain/blockly,nickmain/blockly,jstnhuang/blockly,google/blockly,nickmain/blockly,twodee/blockly,nickmain/blockly,picklesrus/blockly,google/blockly,joshlory/blockly,trinketapp/blockly,jstnhuang/blockly,jstnhuang/blockly,trinketapp/blockly,rachel-fenichel/blockly,trinketapp/blockly,twodee/blockly,twodee/blockly,picklesrus/blockly,google/blockly,mark-friedman/blockly,joshlory/blockly,joshlory/blockly,rachel-fenichel/blockly,google/blockly,google/blockly,google/blockly,mark-friedman/blockly,twodee/blockly