Add a WARNINGS_AS_ERRORS make variable

By default, WARNINGS_AS_ERRORS is set to -Werror to cause erlc to
treat warnings as errors. The WARNINGS_AS_ERRORS variable allows this
to be easily turned off when running make:

    make all WARNINGS_AS_ERRORS=

include.mk

@@ -3,7 +3,9 @@ am__v_ERLC_ = $(am__v_ERLC_$(AM_DEFAULT_VERBOSITY))
 am__v_ERLC_0 = @echo "  ERLC    " $@;
 am__v_ERLC_1 =
 
-ERLC_GENERIC_FLAGS = -Werror  +debug_info $(DEBUG_ERLC_FLAGS)				\
+WARNINGS_AS_ERRORS = -Werror
+
+ERLC_GENERIC_FLAGS = $(WARNINGS_AS_ERRORS) +debug_info $(DEBUG_ERLC_FLAGS)		\
 		     -pa $(top_srcdir) -pa $(top_builddir) -pa $(top_builddir)/ebin	\
 		     -I $(top_srcdir)/include -I $(srcdir)/../include 			\
 		     -I $(top_builddir)/include -I $(builddir)/../include