src/CMakeLists.txt: exclude parts not buildable on FreeBSD

 - FreeBSD does not have keyutils or any kernel type security store
   So using sercert.c is not really relevant here
 - This also excludes building mount.cephfs,
   Next to the fact that this is 100% linux bases mount stuff
 - krbd is the Linux kernel version for RBD based disks, but again
   completely Linux based and porting to FreeBSD, if any, has to follow
   a different route

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>

src/CMakeLists.txt

@@ -696,9 +696,11 @@ install(PROGRAMS
 
 add_subdirectory(bash_completion)
 
-set(parse_secret_files
-  common/secret.c)
-add_library(parse_secret_objs OBJECT ${parse_secret_files})
+if(LINUX)
+  set(parse_secret_files
+    common/secret.c)
+  add_library(parse_secret_objs OBJECT ${parse_secret_files})
+endif()
 
 if(WITH_LIBCEPHFS)
   add_subdirectory(client)
@@ -750,7 +752,9 @@ endif(WITH_LIBCEPHFS)
 
 add_subdirectory(journal)
 
-add_library(krbd_objs OBJECT krbd.cc)
+if(LINUX)
+  add_library(krbd_objs OBJECT krbd.cc)
+endif()
 
 if(${WITH_RBD})
   add_subdirectory(librbd)