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

Committed: https://skia.googlesource.com/skia/+/6b87df2997980218308954ade47df7fdd6680bc7

[tfarina, 2014-10-04 00:32:56]

I easily verified this with:

$ cat macro_expand.cc 
#define SK_BUILD_FOR_LINUX

#if SK_BUILD_FOR_LINUX

#endif

$ cpp macro_expand.cc
# 1 "macro_expand.cc"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "macro_expand.cc"
macro_expand.cc:3:23: error: #if with no expression

Then changing to:
$ cat macro_expand.cc 
#define SK_BUILD_FOR_LINUX

#if defined(SK_BUILD_FOR_LINUX)

#endif

Results in:
$ cpp macro_expand.cc 
# 1 "macro_expand.cc"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "macro_expand.cc"

And also for:
$ cat macro_expand.cc 
#define SK_BUILD_FOR_LINUX 1

#if SK_BUILD_FOR_LINUX

#endif

$ cpp macro_expand.cc 
# 1 "macro_expand.cc"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "macro_expand.cc"

[robertphillips, 2014-10-06 17:36:41]

lgtm.

[tfarina, 2014-10-06 17:37:36]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2014-10-06 17:38:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/628763005/1

[commit-bot: I haz the power, 2014-10-06 17:47:16]

Committed patchset #1 (id:1) as 6b87df2997980218308954ade47df7fdd6680bc7