cmake: Correct CMake checks for LINUX

Some CMake checks were recently switched from UNIX to LINUX to get them
to not apply to macOS/OSX. Since LINUX doesn't seem to be defined,
switch these checks to UNIX AND NOT APPLE.

UI/frontend-plugins/frontend-tools/CMakeLists.txt

@@ -5,7 +5,7 @@ if(APPLE)
 	include_directories(${COCOA})
 endif()
 
-if(LINUX)
+if(UNIX AND NOT APPLE)
 	find_package(X11 REQUIRED)
 	link_libraries(${X11_LIBRARIES})
 	include_directories(${X11_INCLUDE_DIR})