pw_data_link: Only build on Linux

Building on Mac fails due to missing sys/epoll.h which is linux only.

Change-Id: I81e9cfd39a9e44d214574ff622aad3d029f01722
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/193650
Commit-Queue: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Rob Mohr <mohrr@google.com>

pw_data_link/BUILD.gn

diff --git a/pw_data_link/BUILD.gn b/pw_data_link/BUILD.gn
index 7da71a6..247dd1a 100644
--- a/pw_data_link/BUILD.gn
+++ b/pw_data_link/BUILD.gn
@@ -79,20 +79,26 @@
   ]
 }
 
-pw_executable("sample_app") {
-  sources = [ "sample_app.cc" ]
-  deps = [
-    ":data_link",
-    ":data_link_thread",
-    ":server_socket",
-    ":socket_data_link",
-    "$dir_pw_allocator:simple_allocator",
-    "$dir_pw_assert",
-    "$dir_pw_log",
-    "$dir_pw_sync:thread_notification",
-    "$dir_pw_thread:thread",
-    "$dir_pw_thread:thread_core",
-  ]
+# pw_data_link requires Linux sys/epoll.h
+if (host_os == "linux") {
+  pw_executable("sample_app") {
+    sources = [ "sample_app.cc" ]
+    deps = [
+      ":data_link",
+      ":data_link_thread",
+      ":server_socket",
+      ":socket_data_link",
+      "$dir_pw_allocator:simple_allocator",
+      "$dir_pw_assert",
+      "$dir_pw_log",
+      "$dir_pw_sync:thread_notification",
+      "$dir_pw_thread:thread",
+      "$dir_pw_thread:thread_core",
+    ]
+  }
+} else {
+  group("sample_app") {
+  }
 }
 
 pw_test("data_link_test") {
@@ -117,8 +123,13 @@
 }
 
 pw_test_group("tests") {
-  tests = [
-    ":data_link_test",
-    ":socket_data_link_test",
-  ]
+  tests = []
+
+  # pw_data_link requires Linux sys/epoll.h
+  if (host_os == "linux") {
+    tests += [
+      ":data_link_test",
+      ":socket_data_link_test",
+    ]
+  }
 }