Modify --build to be a standalone option (#2294)

- Issue an error when --build is used together with --make
- When given --build, always use GNU Make to perform the build
- Update documentation (examples were good as they were)
- Remove the broken t_flag_build_cmake test

Fixes #2280

src/V3Options.cpp

@@ -593,6 +593,10 @@ void V3Options::notify() {
                 "--xml-only or --E option");
     }
 
+    if (m_build && (m_gmake || m_cmake)) {
+        cmdfl->v3error("--make cannot be used together with --build. Suggest see manual");
+    }
+
     // Make sure at least one make system is enabled
     if (!m_gmake && !m_cmake) m_gmake = true;