build: add --enable-werror

This allows enabling -Werror in a consistent way.  Note that this is
different from just specifiying it in CFLAGS, since that would either
break configure tests (if done on ./configure), or would override
configure's CFLAGS (if done on make).

Using --enable-werror instead provides a new WERROR variable that is
additionally used during make with a consistent set of warning flags.

The tests/ directory is exempt.  (Rationale being, better to have more
tests than pedantically complain about them.)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/buildtest.sh b/buildtest.sh
index 3bc25f2..3dd8ed8 100755
--- a/buildtest.sh
+++ b/buildtest.sh
@@ -4,7 +4,7 @@
 # builds some git commit of Quagga in some different configurations
 # usage: buildtest.sh [commit [configurations...]]
 
-basecfg="--prefix=/usr --enable-user=quagga --enable-group=quagga --enable-vty-group=quagga --enable-configfile-mask=0660 --enable-logfile-mask=0640 --enable-vtysh --sysconfdir=/etc/quagga --enable-exampledir=/etc/quagga/samples --localstatedir=/var/run/quagga --libdir=/usr/lib64/quagga --enable-rtadv --disable-static --enable-isisd --enable-multipath=0 --enable-babeld --enable-pimd"
+basecfg="--prefix=/usr --enable-user=quagga --enable-group=quagga --enable-vty-group=quagga --enable-configfile-mask=0660 --enable-logfile-mask=0640 --enable-vtysh --sysconfdir=/etc/quagga --enable-exampledir=/etc/quagga/samples --localstatedir=/var/run/quagga --libdir=/usr/lib64/quagga --enable-rtadv --disable-static --enable-isisd --enable-multipath=0 --enable-babeld --enable-pimd --enable-werror"
 
 configs_base="gcc|$basecfg"