build: allow system bdb

[div72]

Closes #2663.
Tested with the following script:

#!/usr/bin/env bash

set -e

CLEAN_CLONE="/tmp/gridcoin-$RANDOM"

git clone --depth=1 --single-branch --branch=embedded-bdb https://github.com/div72/Gridcoin-Research.git $CLEAN_CLONE

cp -r $CLEAN_CLONE /tmp/gridcoin-embedded-bdb-in-tree
cp -r $CLEAN_CLONE /tmp/gridcoin-embedded-bdb-out-of-tree
cp -r $CLEAN_CLONE /tmp/gridcoin-system-bdb-in-tree
cp -r $CLEAN_CLONE /tmp/gridcoin-system-bdb-out-of-tree

cd /tmp/gridcoin-embedded-bdb-in-tree
./autogen.sh
./configure
make -j$(nproc --ignore=1)
! ldd src/gridcoinresearchd | grep libdb

cd /tmp/gridcoin-system-bdb-in-tree
./autogen.sh
./configure BDB_LIBS="-ldb_cxx-5.3" BDB_CFLAGS="-I/usr/include/db5.3" --disable-embedded-bdb
make -j$(nproc --ignore=1)
ldd src/gridcoinresearchd | grep libdb

cd /tmp/gridcoin-embedded-bdb-out-of-tree
./autogen.sh
mkdir build && cd build
../configure
make -j$(nproc --ignore=1)
! ldd src/gridcoinresearchd | grep libdb

cd /tmp/gridcoin-system-bdb-out-of-tree
./autogen.sh
mkdir build && cd build
../configure BDB_LIBS="-ldb_cxx-5.3" BDB_CFLAGS="-I/usr/include/db5.3" --disable-embedded-bdb
make -j$(nproc --ignore=1)
ldd src/gridcoinresearchd | grep libdb

rm -rf $CLEAN_CLONE /tmp/gridcoin-embedded-bdb-in-tree /tmp/gridcoin-embedded-bdb-out-of-tree /tmp/gridcoin-system-bdb-in-tree /tmp/gridcoin-system-bdb-out-of-tree

[jamescowens]

ACK