diff --git a/configure.ac b/configure.ac
index ec1d45d2..89d21cf7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -13,6 +13,7 @@ AC_DEFINE([HAVE_PROTOTYPES], 1, [Define if your compiler supports prototypes])
 
 # Checks for programs.
 SAVED_CFLAGS=${CFLAGS}
+SAVED_CPPFLAGS=${CPPFLAGS}
 AC_PROG_CPP
 AC_PROG_CC
 AC_PROG_INSTALL
@@ -30,6 +31,9 @@ if test "x${GCC}" = "xyes"; then
   if test "x${SAVED_CFLAGS}" = "x"; then
     CFLAGS=-O3
   fi
+  if test "x${SAVED_CPPFLAGS}" = "x"; then
+    CPPFLAGS=-Wall
+  fi
 fi
 
 AC_CHECK_DECL([__SUNPRO_C], [SUNCC="yes"], [SUNCC="no"])