Fix build at 2f4232d.

TBR=tsepez@chromium.org,thakis@chromium.org

Review URL: https://codereview.chromium.org/1778893002 .

build/standalone.gypi

diff --git a/build/standalone.gypi b/build/standalone.gypi
index dd3e8e4..6d1d3cb 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -341,14 +341,14 @@
               '-Wno-inconsistent-missing-override',
             ],
           }],
-          ['clang==1 and MSVS_VERSION == "2013"', {
+          ['OS=="win" and clang==1 and MSVS_VERSION == "2013"', {
             'VCCLCompilerTool': {
               'AdditionalOptions': [
                 '-fmsc-version=1800',
               ],
             },
           }],
-          ['clang==1 and MSVS_VERSION == "2015"', {
+          ['OS=="win" and clang==1 and MSVS_VERSION == "2015"', {
             'VCCLCompilerTool': {
               'AdditionalOptions': [
                 '-fmsc-version=1900',