convert plugin functionality into a package

unittest/commands/CMakeLists.txt

@@ -1,6 +1,6 @@
 
 # build LAMMPS plugins, but not on Windows
-if(NOT ${CMAKE_SYSTEM_NAME} STREQUAL "Windows")
+if(NOT ${CMAKE_SYSTEM_NAME} STREQUAL "Windows" AND PKG_PLUGIN)
   ExternalProject_Add(plugins
                       SOURCE_DIR      "${LAMMPS_DIR}/examples/plugins"
                       BINARY_DIR      ${CMAKE_BINARY_DIR}/build-plugins

unittest/commands/test_simple_commands.cpp

@@ -367,6 +367,7 @@ TEST_F(SimpleCommandsTest, Units)
     TEST_FAILURE(".*ERROR: Illegal units command.*", lmp->input->one("units unknown"););
 }
 
+#if defined(LMP_PLUGIN)
 TEST_F(SimpleCommandsTest, Plugin)
 {
 #if defined(__APPLE__)
@@ -437,6 +438,7 @@ TEST_F(SimpleCommandsTest, Plugin)
     if (verbose) std::cout << text;
     ASSERT_THAT(text, MatchesRegex(".*Currently loaded plugins.*"));
 }
+#endif
 
 TEST_F(SimpleCommandsTest, Shell)
 {