refactor LAMMPS plugin building to share more code and to add a demo for ML-PACE

examples/PACKAGES/pace/plugin/CMakeLists.txt

@@ -0,0 +1,36 @@
+##########################################
+# CMake build system for plugin examples.
+# The is meant to be used as a template for plugins that are
+# distributed independent from the LAMMPS package.
+##########################################
+
+cmake_minimum_required(VERSION 3.10)
+
+project(paceplugin VERSION 1.0 LANGUAGES CXX)
+
+set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR})
+include(CheckIncludeFileCXX)
+include(LAMMPSInterfacePlugin)
+include(ML-PACE)
+
+##########################
+# building the plugins
+
+add_library(paceplugin MODULE paceplugin.cpp ${LAMMPS_SOURCE_DIR}/ML-PACE/pair_pace.cpp)
+target_link_libraries(paceplugin PRIVATE pace)
+target_link_libraries(paceplugin PRIVATE lammps)
+target_include_directories(paceplugin PRIVATE ${LAMMPS_SOURCE_DIR}/ML-PACE)
+set_target_properties(paceplugin PROPERTIES PREFIX "" SUFFIX ".so")
+
+# MacOS seems to need this
+if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
+  set_target_properties(paceplugin PROPERTIES LINK_FLAGS "-Wl,-undefined,dynamic_lookup")
+elseif(CMAKE_SYSTEM_NAME STREQUAL "Windows")
+# tell CMake to export all symbols to a .dll on Windows with special case for MinGW cross-compilers
+  set_target_properties(paceplugin PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS TRUE)
+  if(CMAKE_CROSSCOMPILING)
+    set_target_properties(paceplugin  PROPERTIES LINK_FLAGS "-Wl,--export-all-symbols")
+  endif()
+else()
+  set_target_properties(paceplugin PROPERTIES LINK_FLAGS "-rdynamic")
+endif()