Fixes ultrasonic declaration (#328)

Also adds test to ensure this doens't happen again

myRobotCpp/build.gradle

@@ -38,5 +38,37 @@ model {
                 }
             }
         }
+
+        // Shared target used just to check that wpilib is fully linked
+        // Use the static target for normal linking
+        myRobotcppShared(NativeExecutableSpec) {
+            targetPlatform 'arm'
+            binaries.all {
+                tasks.withType(CppCompile) {
+                    addNiLibraryLinks(linker, targetPlatform)
+                    addNetworkTablesLibraryLinks(it, linker, targetPlatform)
+                }
+
+                cppCompiler.args '-pthread', '-Wno-unused-variable'
+                linker.args '-pthread', '-Wno-unused-variable', '-Wl,-rpath,/opt/GenICam_v2_3/bin/Linux_armv7-a'
+            }
+            sources {
+                cpp {
+                    source {
+                        srcDir 'src'
+                        include '**/*.cpp'
+                    }
+                    exportedHeaders {
+                        srcDirs = ['include',
+                                   "${project.athena}/include", "${project.shared}/include",
+                                   "${project.hal}/include/HAL", netTablesInclude, wpiUtilInclude]
+                        include '**/*.h'
+                    }
+
+                    lib project: ':wpilibc', library: 'wpilibc', linkage: 'shared'
+                    lib project: ':hal', library: 'HALAthena', linkage: 'shared'
+                }
+            }
+        }
     }
 }

wpilibc/athena/src/Ultrasonic.cpp

@@ -28,6 +28,7 @@ constexpr double Ultrasonic::kSpeedOfSoundInchesPerSec;
 // automatic round robin mode
 std::atomic<bool> Ultrasonic::m_automaticEnabled{false};
 std::set<Ultrasonic*> Ultrasonic::m_sensors;
+std::thread Ultrasonic::m_thread;
 
 /**
  * Background task that goes through the list of ultrasonic sensors and pings