Patch: Don't install unneeded files into certs directory while building source

[kribby]

Apologies if my wording is a bit confusing. Programming isn't my day job.

I'm working on a Dockerfile for Prosody, using Alpine Linux as the base. I noticed an abnormality when compiling from the source, where some unneeded files are being copied into the /etc/prosody/config/certs directory during a make install. These files are:

/etc/prosody/certs/GNUmakefile
/etc/prosody/certs/localhost.cnf
/etc/prosody/certs/makefile
/etc/prosody/certs/openssl.cnf

I took a look at the GNUmakefile, and I saw that it's copying over everything from the source's certs directory into the final install location, instead of just the generated certs and keys.

The patch I am submitting brings 'GNUmakefile' more in line with how the make install is handled in 'makefile'. The generated example and localhost certificate & key will be the only files installed into the certs directory now. If the --no-example-certs option is specified when running 'configure', nothing will be installed into the certs directory.

Thank You