[gnome-flashback] build: error out if PAM is not found

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 0896db9ca070e9e08f5d7e50b979fc45b8751965
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Wed Mar 18 15:11:51 2020 +0200

    build: error out if PAM is not found
    
    https://gitlab.gnome.org/GNOME/gnome-flashback/issues/40

 configure.ac | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 246d190..af070db 100644
--- a/configure.ac
+++ b/configure.ac
@@ -364,8 +364,13 @@ dnl **************************************************************************
 dnl PAM
 dnl **************************************************************************
 
-AC_CHECK_HEADERS([security/pam_appl.h])
-AC_SEARCH_LIBS([pam_start], [pam])
+AC_CHECK_HEADERS([security/pam_appl.h], [], [
+  AC_MSG_ERROR([PAM development files not found])
+])
+
+AC_SEARCH_LIBS([pam_start], [pam], [], [
+  AC_MSG_ERROR([PAM library not found])
+])
 
 dnl **************************************************************************
 dnl Process .in files