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/ospfclient/Makefile.am b/ospfclient/Makefile.am
index 09d2ba8..c4f0da8 100644
--- a/ospfclient/Makefile.am
+++ b/ospfclient/Makefile.am
@@ -1,6 +1,7 @@
 ## Automake.am for OSPF API client
 
 AM_CPPFLAGS = -I.. -I$(top_srcdir) -I$(top_srcdir)/lib -I$(top_builddir)/lib
+AM_CFLAGS = $(WERROR)
 
 lib_LTLIBRARIES = libospfapiclient.la
 libospfapiclient_la_LDFLAGS = -version-info 0:0:0