Hide the Linux specific parts behind check for headers

Summary:
Only build
  * `virtual_terminal.cpp` in the presence of `linux/vt.h`
  * `fbdev`-backend in the precense of `linux/fb.h`

Test Plan:

Reviewers: #kwin, #freebsd, graesslin, bcooksley

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6847

CMakeLists.txt

@@ -325,6 +325,14 @@ add_feature_info("prctl/procctl tracing control"
 
 configure_file(config-kwin.h.cmake ${CMAKE_CURRENT_BINARY_DIR}/config-kwin.h )
 
+check_include_file("linux/vt.h" HAVE_LINUX_VT_H)
+add_feature_info("linux/vt.h"
+                 HAVE_LINUX_VT_H
+                 "Required for virtual terminal support under wayland")
+check_include_file("linux/fb.h"    HAVE_LINUX_FB_H)
+add_feature_info("linux/fb.h"
+                 HAVE_LINUX_FB_H
+                 "Required for the fbdev backend")
 
 ########### global ###############
 set(kwin_effects_dbus_xml ${CMAKE_CURRENT_SOURCE_DIR}/org.kde.kwin.Effects.xml)
@@ -480,8 +488,13 @@ if(HAVE_INPUT)
         libinput/device.cpp
         libinput/events.cpp
         libinput/libinput_logging.cpp
-        virtual_terminal.cpp
     )
+    if (HAVE_LINUX_VT_H)
+         set(kwin_KDEINIT_SRCS
+             ${kwin_KDEINIT_SRCS}
+             virtual_terminal.cpp
+         )
+    endif()
 endif()
 
 kconfig_add_kcfg_files(kwin_KDEINIT_SRCS settings.kcfgc)

plugins/platforms/CMakeLists.txt

@@ -2,7 +2,9 @@ if(HAVE_INPUT)
     if(HAVE_DRM)
         add_subdirectory(drm)
     endif()
-    add_subdirectory(fbdev)
+    if (HAVE_LINUX_FB_H)
+        add_subdirectory(fbdev)
+    endif()
     if(HAVE_LIBHYBRIS)
         add_subdirectory(hwcomposer)
     endif()