configure: use same CPPFLAGS in kFreeBSD as Linux

046f081b reorganized the CPPFLAGS to no
longer add -D_POSIX_C_SOURCE unconditionally, but only on systems (e.g.,
glibc based ones) that require it.  As kFreeBSD uses glibc, it needs to
be treated similar.

Additionally, _BSD_SOURCE is turned on to enable some additional types
such as caddr_t, which are normally enabled on BSD but not with glibc.

configure

@@ -2487,6 +2487,7 @@ case $target_os in
         enable dos_paths
         ;;
     gnu/kfreebsd)
+        add_cppflags -D_POSIX_C_SOURCE=200112 -D_XOPEN_SOURCE=600 -D_BSD_SOURCE
         ;;
     gnu)
         ;;