Auto merge of #4961 - str4d:leveldb-1.22-prep, r=str4d

Prepare for LevelDB 1.22 update

Cherry-picked from the following upstream PRs:
- bitcoin/bitcoin#7944
- bitcoin/bitcoin#8604
  - Only the first commit (we removed the upstream build documentation).
- bitcoin/bitcoin#12294
  - Only the third commit.
- bitcoin/bitcoin#13659
- bitcoin/bitcoin#16110

configure.ac

@@ -479,15 +479,38 @@ case $host in
      CPPFLAGS="$CPPFLAGS -DMAC_OSX"
      OBJCXXFLAGS="$CXXFLAGS"
      ;;
+   *android*)
+     dnl make sure android stays above linux for hosts like *linux-android*
+     TARGET_OS=android
+     LEVELDB_TARGET_FLAGS="-DOS_ANDROID"
+     ;;
    *linux*)
+     TARGET_OS=linux
      LEVELDB_TARGET_FLAGS="-DOS_LINUX"
      ;;
+   *kfreebsd*)
+     LEVELDB_TARGET_FLAGS="-DOS_KFREEBSD"
+     ;;
    *freebsd*)
      LEVELDB_TARGET_FLAGS="-DOS_FREEBSD"
      ;;
+   *openbsd*)
+     LEVELDB_TARGET_FLAGS="-DOS_OPENBSD"
+     ;;
+   *netbsd*)
+     LEVELDB_TARGET_FLAGS="-DOS_NETBSD"
+     ;;
+   *dragonfly*)
+     LEVELDB_TARGET_FLAGS="-DOS_DRAGONFLYBSD"
+     ;;
+   *solaris*)
+     LEVELDB_TARGET_FLAGS="-DOS_SOLARIS"
+     ;;
+   *hpux*)
+     LEVELDB_TARGET_FLAGS="-DOS_HPUX"
+     ;;
    *)
-     OTHER_OS=`echo ${host_os} | awk '{print toupper($0)}'`
-     LEVELDB_TARGET_FLAGS="-DOS_${OTHER_OS}"
+     AC_MSG_ERROR(Cannot build leveldb for $host. Please file a bug report.)
      ;;
 esac