Use "getconf" to get correct LFS compile and link flags.
* Makefile (GPGME_CFLAGS): Added output of "getconf LFS_CFLAGS".
(GPGME_LIBS): Added output of "getconf LFS_LIBS" and
"getconf LFS_LDFLAGS".
* plugins.d/mandos-client.c: Only define "_LARGEFILE_SOURCE" and
"_FILE_OFFSET_BITS" if they are not
already defined.