Define _WANT_SEMUN for FreeBSD to ensure `union semun` is defined

Build was failing on GhostBSD (FreeBSD-13.0) because `union semun` is switched off by default. Defining _WANT_SEMUN switches it on.

external/db_drivers/CMakeLists.txt

@@ -34,4 +34,6 @@ set(LMDB_INCLUDE "${CMAKE_CURRENT_SOURCE_DIR}/liblmdb" CACHE STRING "LMDB Includ
 set(LMDB_LIBRARY "lmdb" CACHE STRING "LMDB Library name")
 if(CMAKE_SYSTEM_NAME MATCHES "NetBSD")
 	set_property(TARGET lmdb APPEND_STRING PROPERTY COMPILE_FLAGS " -D_SEM_SEMUN_UNDEFINED")
+elseif(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
+        set_property(TARGET lmdb APPEND_STRING PROPERTY COMPILE_FLAGS " -D_WANT_SEMUN")
 endif()