Only set sysroot for MIPS on linux.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index 92e67d59dcf2f06d662ba39839b93a7c861e471b..75cb12e9d28d491ae97381a17ef1da26a9cbd773 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -654,7 +654,7 @@
         ],
         }], # linux_lsb_release=="12.04"
 
-        ['target_arch=="mipsel"', {
+        ['OS=="linux" and target_arch=="mipsel"', {
           'sysroot%': '<!(cd <(DEPTH) && pwd -P)/mipsel-sysroot/sysroot',
           'CXX%': '<!(cd <(DEPTH) && pwd -P)/mipsel-sysroot/bin/mipsel-linux-gnu-gcc',
         }],
@@ -1137,6 +1137,8 @@
             'nacl_untrusted_build%': 0,
             'linux_use_tcmalloc%': 0,
             'linux_breakpad%': 0,
+          }],
+          ['OS=="linux" and target_arch=="mipsel"', {
             'sysroot%': '<(sysroot)',
             'CXX%': '<(CXX)',
           }],