Add environment settings to specify the CFLAGS and LIBS to be used for
BerkeleyDB directly. These will completely by-pass autodetection in the
same way as other similar flags.
```
BDB_CFLAGS C compiler flags for BerkeleyDB, bypasses autodetection
BDB_LIBS Linker flags for BerkeleyDB, bypasses autodetection
```
Implements #3921.