diff --git a/configure.ac b/configure.ac
index 8af9b24..6a4853e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -510,7 +510,7 @@ AC_ARG_ENABLE(squat,
     [AS_HELP_STRING([--disable-squat], [disable Squat support])],,[enable_squat="yes";])
 
 if test "$enable_squat" != "no"; then
-    AC_DEFINE(USE_SQUAT,[],[Build in Sieve support?])
+    AC_DEFINE(USE_SQUAT,[],[Build in Squat support?])
 fi
 
 AM_CONDITIONAL([USE_SQUAT], [test "${enable_squat}" != "no"])
@@ -609,6 +609,14 @@ if test "$enable_sieve" != "no"; then
         AC_PROG_YACC
         AM_PROG_LEX
 
+        if test -z "$ac_cv_prog_YACC"; then
+            AC_MSG_ERROR([Sieve requires bison/byacc/yacc, but none is installed])
+        fi
+
+        if test -z "$ac_cv_prog_LEX"; then
+            AC_MSG_ERROR([Sieve requires flex/lex, but none is installed])
+        fi
+
         if test "$enable_pcre" != "no"; then
             AC_CHECK_HEADER(pcreposix.h)
             if test "$ac_cv_header_pcreposix_h" = "yes"; then
@@ -1499,7 +1507,7 @@ dnl
                         AC_DEFINE(HAVE_RSCALE,[], [Build RSCALE support into httpd?])
                         LIBS="$LIBS ${ICU4C_LIBS}"
                         ],
-                        AC_MSG_WARN([Need ICU4C for RSCALE support in httpd]))
+                        AC_MSG_ERROR([Need ICU4C for RSCALE support in httpd]))
                 ],
                 AC_MSG_WARN([Your version of libical can not support non-gregorian recurrences.  Consider upgrading to libical >= 2.0]))
 
@@ -1586,12 +1594,12 @@ if test -z "$with_com_err"; then
         ])
 
         if test "${with_com_err}" = "no"; then
-          AC_MSG_WARN([com_err is required; included version will be used.])
+          AC_MSG_NOTICE([com_err is required; included version will be used.])
           with_com_err="yes"
         fi
         if test "${COMPILE_ET}" = "no compile et" -o "${COMPILE_ET}" = ""; then
-          AC_MSG_WARN([Parts of com_err distribuion were found, but not compile_et.])
-          AC_MSG_WARN([Will build com_err from included sources.])
+          AC_MSG_NOTICE([Parts of com_err distribuion were found, but not compile_et.])
+          AC_MSG_NOTICE([Will build com_err from included sources.])
           with_com_err="yes" # build it ourselves
         fi
 fi