build: Add options to override BDB cflags/libs

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.

build-aux/m4/bitcoin_find_bdb48.m4

@@ -3,9 +3,12 @@ dnl Distributed under the MIT software license, see the accompanying
 dnl file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 AC_DEFUN([BITCOIN_FIND_BDB48],[
+  AC_ARG_VAR(BDB_CFLAGS, [C compiler flags for BerkeleyDB, bypasses autodetection])
+  AC_ARG_VAR(BDB_LIBS, [Linker flags for BerkeleyDB, bypasses autodetection])
+
+  if test "x$BDB_CFLAGS" = "x"; then
     AC_MSG_CHECKING([for Berkeley DB C++ headers])
     BDB_CPPFLAGS=
-  BDB_LIBS=
     bdbpath=X
     bdb48path=X
     bdbdirlist=
@@ -54,8 +57,12 @@ AC_DEFUN([BITCOIN_FIND_BDB48],[
       BITCOIN_SUBDIR_TO_INCLUDE(BDB_CPPFLAGS,[${bdb48path}],db_cxx)
       bdbpath="${bdb48path}"
     fi
+  else
+    BDB_CPPFLAGS=${BDB_CFLAGS}
+  fi
   AC_SUBST(BDB_CPPFLAGS)
   
+  if test "x$BDB_LIBS" = "x"; then
     # TODO: Ideally this could find the library version and make sure it matches the headers being used
     for searchlib in db_cxx-4.8 db_cxx; do
       AC_CHECK_LIB([$searchlib],[main],[
@@ -66,5 +73,6 @@ AC_DEFUN([BITCOIN_FIND_BDB48],[
     if test "x$BDB_LIBS" = "x"; then
         AC_MSG_ERROR([libdb_cxx missing, ]AC_PACKAGE_NAME[ requires this library for wallet functionality (--disable-wallet to disable wallet functionality)])
     fi
+  fi
   AC_SUBST(BDB_LIBS)
 ])