configure: Improve error message when pkg-config is missing.

Until now, when pkg-config is missing, Autoconf emitted this error:
    error: possibly undefined macro: PKG_CHECK_MODULES
This commit changes the message to:
    error: Please install pkg-config.
This should be easier for users to interpret.

Suggested-by: Ethan Jackson <ethan@nicira.com>

diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
index 73db4bb..e9edc49 100644 (file)
--- a/m4/openvswitch.m4
+++ b/m4/openvswitch.m4
@@ -92,8 +92,7 @@ AC_DEFUN([OVS_CHECK_OPENSSL],
      [ssl=check])
 
    if test "$ssl" != false; then
-       dnl Make sure that pkg-config is installed.
-       m4_pattern_forbid([PKG_CHECK_MODULES])
+       m4_ifndef([PKG_CHECK_MODULES], [m4_fatal([Please install pkg-config.])])
        PKG_CHECK_MODULES([SSL], [openssl], 
          [HAVE_OPENSSL=yes],
          [HAVE_OPENSSL=no