Make Xrandr not implicitly required when x11 is used

The previous logic made xrandr a required header whenever x11 was
available which doesn't honestly convey build requirements to users.

This commit add BUILD_WSI_XLIB_XRANDR_SUPPORT as a build option
that defaults to ON (for platforms which support x11) so the xrandr
requirement can be configured by users directly.

CMakeLists.txt

@@ -111,6 +111,7 @@ elseif(APPLE)
 elseif(CMAKE_SYSTEM_NAME MATCHES "Linux|BSD|DragonFly|GNU")
     option(BUILD_WSI_XCB_SUPPORT "Build XCB WSI support" ON)
     option(BUILD_WSI_XLIB_SUPPORT "Build Xlib WSI support" ON)
+    option(BUILD_WSI_XLIB_XRANDR_SUPPORT "Build X11 Xrandr WSI support" ON)
     option(BUILD_WSI_WAYLAND_SUPPORT "Build Wayland WSI support" ON)
     option(BUILD_WSI_DIRECTFB_SUPPORT "Build DirectFB WSI support" OFF)
 
@@ -125,8 +126,14 @@ elseif(CMAKE_SYSTEM_NAME MATCHES "Linux|BSD|DragonFly|GNU")
     if(BUILD_WSI_XLIB_SUPPORT)
         pkg_check_modules(X11 REQUIRED QUIET IMPORTED_TARGET x11)
         pkg_get_variable(XLIB_INCLUDE_DIRS x11 includedir)
-        target_compile_definitions(platform_wsi INTERFACE VK_USE_PLATFORM_XLIB_KHR VK_USE_PLATFORM_XLIB_XRANDR_EXT)
+        target_compile_definitions(platform_wsi INTERFACE VK_USE_PLATFORM_XLIB_KHR)
         target_include_directories(platform_wsi INTERFACE ${XLIB_INCLUDE_DIRS})
+        if(BUILD_WSI_XLIB_XRANDR_SUPPORT)
+            pkg_check_modules(XRANDR REQUIRED QUIET IMPORTED_TARGET xrandr)
+            pkg_get_variable(XLIB_XRANDR_INCLUDE_DIRS xrandr includedir)
+            target_compile_definitions(platform_wsi INTERFACE VK_USE_PLATFORM_XLIB_XRANDR_EXT)
+            target_include_directories(platform_wsi INTERFACE ${XLIB_XRANDR_INCLUDE_DIRS})
+        endif()
     endif()
     if(BUILD_WSI_WAYLAND_SUPPORT)
         target_compile_definitions(platform_wsi INTERFACE VK_USE_PLATFORM_WAYLAND_KHR)