Added "db4_cxx" as this is the name used in NetBSD for the Berkeley DB library. Without this additional base filename alternative, the "configure" script will fail to find this library and compilation will not succeed.
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.
Jon Layton