bitcoin-config.h moved, but the old file is likely to still exist when
reconfiguring or switching branches. This would've caused files to not rebuild
correctly, and other strange problems.
Make the path explicit so that the old one cannot be found.
Core libs use config/bitcoin-config.h.
Libs (like crypto) which don't want access to bitcoin's headers continue
to use -Iconfig and #include bitcoin-config.h.
glibc/libstdc++ have added new symbols in later releases. When running a new
binary against an older glibc, the run-time linker is unable to resolve the
new symbols and the binary refuses to run.
This can be fixed by adding our own versions of those functions, so that the
build-time linker does not emit undefined symbols for them.
This enables our binary releases to work on older Linux distros, while not
incurring the downsides of a fully static binary.
