Don't use clip_none as fallback to avoid silent errors

This might result in an unexpected behavior (using the clip_none impl)
in platforms which clip supports.

CMakeLists.txt

@@ -64,37 +64,34 @@ elseif(APPLE)
     target_compile_options(clip PRIVATE -fobjc-arc)
   endif()
 
-  find_library(COCOA_LIBRARY Cocoa)
-  if(COCOA_LIBRARY)
-    target_sources(clip PRIVATE clip_osx.mm)
-    target_link_libraries(clip ${COCOA_LIBRARY})
-  else()
-    target_sources(clip PRIVATE clip_none.cpp)
-  endif()
+  find_library(COCOA_LIBRARY REQUIRED Cocoa)
+
+  target_sources(clip PRIVATE clip_osx.mm)
+  target_link_libraries(clip ${COCOA_LIBRARY})
 elseif(UNIX)
   include(CheckIncludeFiles)
   check_include_files(xcb/xcb.h HAVE_XCB_XLIB_H)
 
-  if(HAVE_XCB_XLIB_H)
-    target_compile_definitions(clip PRIVATE -DHAVE_XCB_XLIB_H)
-    target_link_libraries(clip xcb pthread)
-
-    if(CLIP_ENABLE_IMAGE AND CLIP_X11_WITH_PNG)
-      check_include_files(png.h HAVE_PNG_H)
-      if(CLIP_X11_PNG_LIBRARY)
-        set(PNG_LIBRARY ${CLIP_X11_PNG_LIBRARY})
-      else()
-        find_library(PNG_LIBRARY png)
-      endif()
-      if(HAVE_PNG_H AND PNG_LIBRARY)
-        target_compile_definitions(clip PRIVATE -DHAVE_PNG_H)
-      endif()
-      target_link_libraries(clip ${PNG_LIBRARY})
+  if(NOT HAVE_XCB_XLIB_H)
+    message(FATAL_ERROR "xcb/xcb.h not found, install libxcb-dev package")
+  endif()
+
+  target_compile_definitions(clip PRIVATE -DHAVE_XCB_XLIB_H)
+  target_link_libraries(clip xcb pthread)
+
+  if(CLIP_ENABLE_IMAGE AND CLIP_X11_WITH_PNG)
+    check_include_files(png.h HAVE_PNG_H)
+    if(CLIP_X11_PNG_LIBRARY)
+      set(PNG_LIBRARY ${CLIP_X11_PNG_LIBRARY})
+    else()
+      find_library(PNG_LIBRARY png)
+    endif()
+    if(HAVE_PNG_H AND PNG_LIBRARY)
+      target_compile_definitions(clip PRIVATE -DHAVE_PNG_H)
     endif()
-    target_sources(clip PRIVATE clip_x11.cpp)
-  else()
-    target_sources(clip PRIVATE clip_none.cpp)
+    target_link_libraries(clip ${PNG_LIBRARY})
   endif()
+  target_sources(clip PRIVATE clip_x11.cpp)
 else()
   target_sources(clip PRIVATE clip_none.cpp)
 endif()