build: fix build weirdness after 54372482.

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.
Merge pull request #4395

f3967bc build: fix build weirdness after 54372482. (Cory Fields)