build: force -Werror for distcheck target

The 'make distcheck' target validates that a tarball builds and
is ready for release. We expect that libvirt builds cleanly on
all supported platforms, so we should be enabling -Werror when
running distcheck.

This ensures that our CI systems in turn also use -Werror.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

Makefile.am

@@ -19,6 +19,10 @@
 LCOV = lcov
 GENHTML = genhtml
 
+# when building from tarball -Werror isn't auto enabled
+# so force it explicitly
+DISTCHECK_CONFIGURE_FLAGS = --enable-werror
+
 SUBDIRS = . gnulib/lib include/libvirt src tools docs gnulib/tests \
   tests po examples