Don't try to build the documentation if doxygen isn't found
Don't try to build the documentation if doxygen isn't found

Making all in doc
make[2]: Entering directory `/home/xhh/graphics/ex/vaapi/libva/doc'
enable_docs=no Doxyfile
/bin/bash: Doxyfile: command not found
make[2]: *** [html-out/index.html] Error 127
make[2]: Leaving directory `/home/xhh/graphics/ex/vaapi/libva/doc'

Signed-off-by: Xiang, Haihao <8d677183e5993577d4fa73a419b76987913292d6@intel.com>

android-ia/hardware_intel_libva,android-ia/hardware_intel_libva,CyanogenMod/android_hardware_intel_libva,CyanogenMod/android_hardware_intel_libva,halleyzhao/libva-fdo,halleyzhao/libva-fdo,CyanogenMod/android_hardware_intel_libva,android-ia/hardware_intel_libva,halleyzhao/libva-fdo