autotools: fails build when need but can not find openssl

diff --git a/m4/libevent_openssl.m4 b/m4/libevent_openssl.m4
index 19811981e91579434cdbab87a41cea19ea6317a1..a5ea67620067fb7301b0f420152300e95ef8a2b4 100644 (file)
--- a/m4/libevent_openssl.m4
+++ b/m4/libevent_openssl.m4
@@ -47,6 +47,11 @@ case "$enable_openssl" in
     AC_SUBST(OPENSSL_LIBS)
     case "$have_openssl" in
      yes)  AC_DEFINE(HAVE_OPENSSL, 1, [Define if the system has openssl]) ;;
+     *) AC_MSG_ERROR([openssl is a must but can not be found. You should add the \
+directory containing `openssl.pc' to the `PKG_CONFIG_PATH' environment variable, \
+or set `CFLAGS' and `LDFLAGS' directly for openssl, or use `--disable-openssl' \
+to disable support for openssl encryption])
+       ;;
     esac
     ;;
 esac