On cygwin, <rpc/rpc.h> lives in a different directory than
/usr/include, so anything that uses it must modify CFLAGS.  This
previously tripped up just 'make check', but now that we build
all test programs unconditionally, it also trips up 'make'.

* tests/Makefile.am (virnetmessagetest_CFLAGS): Find rpc headers.