configure: fix definition of HAVE_LMDB

configure.ac

@@ -314,7 +314,7 @@ AM_CONDITIONAL([HAVE_DNSTAP], test "$opt_dnstap" != "no")
 
 dnl Check for LMDB
 KNOT_CHECK_HEADER([lmdb], [LMDB], [auto], [lmdb.h], [], [-llmdb])
-AM_CONDITIONAL([HAVE_LMDB], [test "$enable_lmdb" = yes])
+AS_IF([test "$enable_lmdb" = yes], [AC_DEFINE([HAVE_LMDB], [1], [Define to 1 to enable LMDB support])])
 
 AC_SEARCH_LIBS([pow], [m])
 AC_SEARCH_LIBS([pthread_create], [pthread], [], [AC_MSG_ERROR([pthreads not found])])