build: improve missing boost error reporting
doc: Add note about memory reqs for compilation
qt: Don't require db_cxx.h when wallet disabled

Fix #3978.
Merge pull request #3986

65adc3a qt: Don't require db_cxx.h when wallet disabled (Wladimir J. van der Laan)
4babd08 doc: Add note about memory reqs for compilation (Wladimir J. van der Laan)
25333a2 build: improve missing boost error reporting (Wladimir J. van der Laan)