configure: Make autoconf fail if libtool is not installed.

Otherwise users get an error later like:
    ./configure: line 5093: syntax error near unexpected token
    `disable-shared' ./configure: line 5093: `LT_INIT(disable-shared)'
It's probably friendlier to make configuration fail earlier.

Signed-off-by: Ben Pfaff <blp@nicira.com>

diff --git a/configure.ac b/configure.ac
index f6a11f0..6a999d0 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -41,6 +41,8 @@ AC_C_BIGENDIAN
 AC_SYS_LARGEFILE
 
 LT_INIT([disable-shared])
+m4_pattern_forbid([LT_INIT]) dnl Make autoconf fail if libtool is missing.
+
 AC_SEARCH_LIBS([pow], [m])
 AC_SEARCH_LIBS([clock_gettime], [rt])
 AC_SEARCH_LIBS([timer_create], [rt])