Require ABI compatibility with VRT 6.0.

Includes a workaround for the problem in Varnish issue #2508.

src/Makefile.am

@@ -34,8 +34,11 @@ vmod_re2.lo: $(nodist_libvmod_re2_la_SOURCES)
 
 vcc_if.h vmod_re2.rst vmod_re2.man.rst: vcc_if.c
 
+# The sed command fixes the bug concerning $ABI vrt that is addressed
+# by Varnish issue #2508.
 vcc_if.c: vmod_re2.vcc
 	$(AM_V_VMODTOOL) $(PYTHON) $(VMODTOOL) -o vcc_if $(srcdir)/vmod_re2.vcc
+	sed -i '/vrt_major/s/0/6/' vcc_if.c
 
 vmod_re2.3: vmod_re2.man.rst
 	$(AM_V_GEN) $(RST2MAN) vmod_re2.man.rst vmod_re2.3

src/vmod_re2.vcc

@@ -46,6 +46,8 @@ $Module re2 3 Varnish Module for access to the Google RE2 regular expression eng
   # VMOD version
   STRING re2.version()
 
+$ABI vrt
+
 DESCRIPTION
 ===========