misc: only build with lto if explicitly enabled

Change the LTO default to off from probe because it wastes build time on
developer machines.  Anyone who really wants it for release builds or
whatever can still turn it on.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>

diff --git a/configure.ac b/configure.ac
index 2448416..38e5426 100644
--- a/configure.ac
+++ b/configure.ac
@@ -91,8 +91,8 @@
 AC_SUBST(enable_threadsan)
 
 AC_ARG_ENABLE(lto,
-[ --enable-lto=[yes/no]      Enable link time optimization (LTO) [default=probe]],,
-	enable_lto=probe)
+[ --enable-lto=[yes/no]      Enable link time optimization (LTO) [default=no]],,
+	enable_lto=no)
 AC_SUBST(enable_lto)
 
 # Enable xfs_scrub build