Allow users to disable -Werror at configure time

Some users and distro packagers might not want to build with -Werror,
this allows them to disable using -Werror but enables it in the default
case so as to maintain the current behavior.

configure.ac

@@ -11,8 +11,14 @@ AC_CANONICAL_HOST
 AM_CONDITIONAL(LD_ISCSI,
                [expr "$host_os" : linux > /dev/null 2>&1])
 
+AC_ARG_ENABLE([werror], [AS_HELP_STRING([--disable-werror],
+              [Disables building with -Werror by default])])
+
 if test "$ac_cv_prog_gcc" = yes; then
-   WARN_CFLAGS="-Wall -W -Wshadow -Wstrict-prototypes -Wpointer-arith -Wcast-align -Wwrite-strings -Werror -Wno-strict-aliasing"
+   WARN_CFLAGS="-Wall -W -Wshadow -Wstrict-prototypes -Wpointer-arith -Wcast-align -Wwrite-strings -Wno-strict-aliasing"
+   if test "x$enable_werror" != "xno"; then
+       WARN_CFLAGS="$WARN_CFLAGS -Werror"
+   fi
 fi
 AC_SUBST(WARN_CFLAGS)