Fix usage of SK_BUILD_* defines.

Fix usage of SK_BUILD_* defines.

Since we just 'define' them, but not attribute anything to them, like
'1' for example, cpp expands it to nothing and that breaks the "#if"
clauses.

To fix that, uses "#if defined(...)" which will correctly check if your
macro name was defined or not.

BUG=skia:2850
TEST=make most
R=robertphillips@google.com

Review URL: https://codereview.chromium.org/628763005