build: Need options to specify BDB location #3921

Many people compile BerkeleyDb 4.8 themselves and understandably don't want to install it system-wide.

I'd suggest the following names:

--with-bdb-libdir=
--with-bdb-includedir=

One can always override LDFLAGS and CFLAGS to find them manually, don't forget that you need to manualy specify optimization flags in that case ie:

./configure LDFLAGS="-L/path/to/libs" CPPFLAGS="-I/path/to/includes -O2"

But it'd be easier if there was a flag for this as it is very common now.

Alternative would be BDB_CFLAGS and BDB_LIBS, analogous to SSL_CFLAGS and SSL_LIBS.

@luke-jr: another one for bitcoin_find_bdb48.m4. Again, I'll take this on, just giving you first dibs.

I'm trying to build Bitcoin Core on FreeBSD and would be very much helped by this. Also in our build guides we routinely override LDFLAGS/CXXFLAGS for this purpose, which shouldn't be necessary if there was an option or a specific env variable for this:

• https://github.com/bitcoin/bitcoin/blob/master/doc/build-unix.md#berkeley-db
• https://github.com/bitcoin/bitcoin/blob/master/doc/build-openbsd.md#building-bitcoin-core

Anyone up for this? I can try to do it myself but I don't know that much about m4 and will probably come up with a hacky solution.

Fixed by #9705