CMake/Linux: Add libharu to platform_linux.cmake

Since GPencil changes depending on libharu may be committed to master
before SVN libraries for all platforms are in place, avoid build issues.
Extension of D9928.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D10280

build_files/cmake/platform/platform_unix.cmake

@@ -470,6 +470,14 @@ if(WITH_POTRACE)
   endif()
 endif()
 
+if(WITH_HARU)
+  find_package_wrapper(Haru)
+  if(NOT HARU_FOUND)
+    message(WARNING "Haru not found, disabling WITH_HARU")
+    set(WITH_HARU OFF)
+  endif()
+endif()
+
 if(EXISTS ${LIBDIR})
   without_system_libs_end()
 endif()