build : fix CPPFLAGS for libbitcoin_cli
tests: fix test-runner for osx. Closes ##4708

Use the more portable $$ rather than $BASHPID
Merge pull request #4710

9322f1a tests: fix test-runner for osx. Closes ##4708 (Cory Fields)
Merge pull request #4707

a65668d build : fix CPPFLAGS for libbitcoin_cli (randy-waterhouse)
Add travis CI builds.
Tidy up pkg.m4 into src/m4. Add minimal bitcoin.pc.in.
Set-up for multi compiler builds.