kFreeBSD: Do not unset LINUX variable

kFreeBSD is a Debian GNU distribution with a kernel from FreeBSD,
and should be marked as LINUX.

Modules/Platform/kFreeBSD.cmake

@@ -1,5 +1,5 @@
-# kFreeBSD looks just like Linux.
+# kFreeBSD is a Debian GNU distribution with a kernel from FreeBSD,
+# and should be marked as LINUX
 include(Platform/Linux)
-unset(LINUX)
 
 set(CMAKE_LIBRARY_ARCHITECTURE_REGEX "[a-z0-9_]+(-[a-z0-9_]+)?-kfreebsd-gnu[a-z0-9_]*")