Use relative paths instead of absolute paths in protoc calls #7788

When comparing two separate build logs (both being done in its own, different directory), the log line containing protoc shines in the diff:

$ diff -u 7774.log  master.log | grep protoc
-/opt/local/bin/protoc --cpp_out=qt --proto_path=/private/tmp/bitcoin-7774/src/qt /private/tmp/bitcoin-7774/src/qt/paymentrequest.proto
+/opt/local/bin/protoc --cpp_out=qt --proto_path=/private/tmp/bitcoin-master/src/qt /private/tmp/bitcoin-master/src/qt/paymentrequest.proto
$ 

There is no reason to use absolute paths here.

After this change, the protoc command line is always

/opt/local/bin/protoc --cpp_out=qt --proto_path=qt/ qt/paymentrequest.proto

@theuni Please have a look.

Travis failure is unrelated to this change.

I think this should be ok. I suspect it was absolute as part of the initial recursive buildsystem.

Please make it ... --proto_path=$(<D) $< though, rather than using "dir".

Done. Thanks.

So after this, the remaining changes/diffs are

-=== configuring in src/univalue (/tmp/bitcoin-master/src/univalue)
+=== configuring in src/univalue (/tmp/bitcoin-7788/src/univalue)

for univalue and secp265k1 directories.

Thanks. ACK.

Don't think we can do anything about the configure subdirs, the pwd is hard-coded.

@theuni yes, I can live with it and ignore such lines.

Nice, makes the line a few characters shorter too. utACK 0087f26