Move db_drivers/ to external/

Also change LMDB Cmake variables to CACHE rather than upgrading them
through several parent scopes.

CMakeLists.txt

@@ -174,9 +174,6 @@ link_directories(${UNBOUND_LIBRARY_DIRS})
 # Final setup for rapidjson
 include_directories(external/rapidjson)
 
-
-add_subdirectory(db_drivers)
-
 # Final setup for liblmdb
 include_directories(${LMDB_INCLUDE})
 

external/CMakeLists.txt

@@ -97,3 +97,5 @@ else()
     die("Found libunbound includes, but could not find libunbound library. Please make sure you have installed libunbound or libunbound-dev or the equivalent")
   endif()
 endif()
+
+add_subdirectory(db_drivers)

external/db_drivers/CMakeLists.txt

@@ -30,6 +30,6 @@
 # critical a consensus component to rely on dynamically linked libraries
 message(STATUS "Using ${ARCH_WIDTH}-bit LMDB from source tree")
 add_subdirectory(liblmdb${ARCH_WIDTH})
-set(LMDB_INCLUDE "${CMAKE_CURRENT_SOURCE_DIR}/liblmdb${ARCH_WIDTH}" PARENT_SCOPE)
+set(LMDB_INCLUDE "${CMAKE_CURRENT_SOURCE_DIR}/liblmdb${ARCH_WIDTH}" CACHE STRING "LMDB Include path")
 
-set(LMDB_LIBRARY "lmdb" PARENT_SCOPE)
+set(LMDB_LIBRARY "lmdb" CACHE STRING "LMDB Library name")