build failure with glibc >= 2.26

[Eray Aslan]

Sun RPC was removed from glibc-2.26 [1] resulting in

/bin/sh ../../../libtool  --tag=CC   --mode=link x86_64-pc-linux-gnu-gcc  -std=gnu99 -march=native -O2 -pipe -fstack-protector-strong -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -Wall -W -Wmissing-prototypes -Wmissing-declarations -Wpointer-arith -Wchar-subscripts -Wformat=2 -Wbad-function-cast -fno-builtin-strftime -Wstrict-aliasing=2   -module -avoid-version  -Wl,-O1 -Wl,--as-needed  -o lib10_doveadm_quota_plugin.la -rpath /usr/lib64/dovecot/doveadm doveadm-quota.lo
rquota_xdr.c:4:10: fatal error: rpc/rpc.h: No such file or directory
 #include <rpc/rpc.h>
          ^~~~~~~~~~~
compilation terminated.
make[4]: *** [Makefile:826: rquota_xdr.lo] Error 1

during build.  Apparently, rquota code needs some love.

This is dovecot-2.3.2 but 2.2 branch should also result in the same
error during make.

-- 
Eray

[1] https://fedoraproject.org/wiki/Changes/SunRPCRemoval