Apple's ranlib warns about protocol_openssl.c

Hi,

Apple's ranlib doesn't like empty translation units[1], but
protocol_openssl.c doesn't define any symbols (unless you have an
ancient EOL'd openssl), so longfin and CI say:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib:
file: libpgcommon.a(protocol_openssl.o) has no symbols

I guess we still can't switch to (Apple) libtool.  Maybe configure
should be doing a test and adding it to LIBOBJS or a similar variable
only if necessary, or something like that?

[1] https://www.postgresql.org/message-id/flat/28521.1426352337%40sss.pgh.pa.us