refactor(cmake): split CMakeLists into modules (#1587)

cmake/packaging/unix.cmake

@@ -0,0 +1,18 @@
+# unix specific packaging
+# put anything here that applies to both linux and macos
+
+# return here if building a macos package
+if(SUNSHINE_PACKAGE_MACOS)
+    return()
+endif()
+
+# Installation destination dir
+set(CPACK_SET_DESTDIR true)
+if(NOT CMAKE_INSTALL_PREFIX)
+    set(CMAKE_INSTALL_PREFIX "/usr/share/sunshine")
+endif()
+
+install(TARGETS sunshine RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")
+
+install(DIRECTORY "${SUNSHINE_SOURCE_ASSETS_DIR}/common/assets/"
+        DESTINATION "${SUNSHINE_ASSETS_DIR}")