not compile leveldb if the lib is not installed on the computer

configure.ac

@@ -1489,8 +1489,10 @@ AC_DEFINE_UNQUOTED(STARPU_MAXIMPLEMENTATIONS, [$maximplementations],
 
 AC_LANG_PUSH([C++])
 AC_CHECK_HEADERS([leveldb/db.h], [AC_DEFINE([STARPU_HAVE_LEVELDB_H], [1], [Define to 1 if you have the <leveldb/db.h> header file.])])
-STARPU_HAVE_LIBRARY(LEVELDB, [leveldb])
 AC_LANG_POP([C++])
+STARPU_HAVE_LIBRARY(LEVELDB, [leveldb], starpu_leveldb_bool=xno, starpu_leveldb_bool=xyes)
+
+AM_CONDITIONAL(STARPU_HAVE_LEVELDB, test x$starpu_leveldb_bool = xyes)
 
 ###############################################################################
 #                                                                             #

src/Makefile.am

@@ -161,7 +161,6 @@ libstarpu_@STARPU_EFFECTIVE_VERSION@_la_SOURCES = 						\
 	core/dependencies/data_concurrency.c			\
 	core/disk_ops/disk_stdio.c				\
 	core/disk_ops/disk_unistd.c                             \
-	core/disk_ops/disk_leveldb.cpp				\
 	core/disk_ops/unistd/disk_unistd_global.c		\
 	core/perfmodel/perfmodel_history.c			\
 	core/perfmodel/perfmodel_bus.c				\
@@ -273,7 +272,9 @@ if STARPU_LINUX_SYS
 libstarpu_@STARPU_EFFECTIVE_VERSION@_la_SOURCES += core/disk_ops/disk_unistd_o_direct.c
 endif
 
-
+if STARPU_HAVE_LEVELDB
+libstarpu_@STARPU_EFFECTIVE_VERSION@_la_SOURCES += core/disk_ops/disk_leveldb.cpp
+endif
 
 #########################################
 #										#