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

cmake/compile_definitions/unix.cmake

@@ -0,0 +1,9 @@
+# unix specific compile definitions
+# put anything here that applies to both linux and macos
+
+list(APPEND SUNSHINE_EXTERNAL_LIBRARIES Boost::log)
+
+# add install prefix to assets path if not already there
+if(NOT SUNSHINE_ASSETS_DIR MATCHES "^${CMAKE_INSTALL_PREFIX}")
+    set(SUNSHINE_ASSETS_DIR "${CMAKE_INSTALL_PREFIX}/${SUNSHINE_ASSETS_DIR}")
+endif()