[build] Refactor and simplify build system

* Main gyp files are now standardized as platform/<platform>/platform.gyp.
* Each platform gyp file defines appropriate loop_lib and headless_lib variables.
* Each platform gyp file includes mbgl.gypi, which defines base targets which may be useful to all platforms.
* CI targets are consistent across platforms: `make $(PLATFORM) && make test-$(PLATFORM)`.
* Renamed the "linux" test app to "glfw". It's now built in OS X CI.
* Android build flakiness is fixed.
* iOS CI builds the bench and iosapp targets.
* Mesa version is now in one place.
* CI scripts use bash "strict mode" and correct error handling.
* All build output goes to the build directory.
* Removed vestigial iOS/OS X/Android Travis scripts.

test/storage/http_error.cpp

@@ -69,17 +69,6 @@ TEST_F(Storage, TEST_REQUIRES_SERVER(HTTPConnectionError)) {
         EXPECT_GT(wait + 0.2, duration) << "Backoff timer fired too late";
         ASSERT_NE(nullptr, res.error);
         EXPECT_EQ(Response::Error::Reason::Connection, res.error->reason);
-#ifdef MBGL_HTTP_NSURL
-        EXPECT_TRUE(res.error->message ==
-                     "The operation couldn’t be completed. (NSURLErrorDomain error -1004.)" ||
-                 res.error->message == "Could not connect to the server.")
-         << "Full message is: \"" << res.error->message << "\"";
-#elif MBGL_HTTP_CURL
-        const std::string prefix { "Couldn't connect to server: " };
-        EXPECT_STREQ(prefix.c_str(), res.error->message.substr(0, prefix.size()).c_str()) << "Full message is: \"" << res.error->message << "\"";
-#else
-        FAIL();
-#endif
         ASSERT_FALSE(res.data.get());
         EXPECT_FALSE(bool(res.expires));
         EXPECT_FALSE(bool(res.modified));