configure checks if RE2 allows a NULL pointer for the vector in

Set::Match(); this makes the VMOD compatible with RE2 since at
least version 2015-05-01.

configure.ac

@@ -81,6 +81,33 @@ AC_PATH_PROG([VARNISHD], [varnishd], [],
 
 PKG_CHECK_MODULES([RE2], [re2])
 
+# RE2 versions up to 2016-03-01 require a pointer to vector<int> in
+# Set::Match(), to identify the regex that was matched. Since commit
+# df7a2dc in re2, the pointer may be NULL, if we just want to know
+# whether there was a match. This check tests for that feature.
+# Note: the test may cause a core dump if it fails.
+AC_LANG(C++)
+SAVE_CXXFLAGS="$CXXFLAGS"
+SAVE_LDFLAGS="$LDFLAGS"
+CXXFLAGS+=" -std=c++11"
+LDFLAGS+=" -lre2"
+AC_RUN_IFELSE(
+	[AC_LANG_SOURCE([[
+#include "re2/set.h"
+main() {
+       re2::RE2::Set s(re2::RE2::DefaultOptions, re2::RE2::UNANCHORED);
+       s.Add("", NULL);
+       s.Compile();
+       s.Match("", NULL);
+}
+	]])],
+	[AC_DEFINE([HAVE_SET_MATCH_NULL_VECTOR], [1],
+	           [Define to 1 if RE2 Set::Match() permits a NULL vector])]
+	)
+CXXFLAGS="$SAVE_CXXFLAGS"
+LDFLAGS="$SAVE_LDFLAGS"
+AC_LANG(C)
+
 # --enable-stack-protector
 AC_ARG_ENABLE(stack-protector,
        AS_HELP_STRING([--enable-stack-protector],[enable stack protector (default is YES)]),

src/vre2/vre2set.cpp

@@ -27,6 +27,7 @@
  *
  */
 
+#include "config.h"
 #include "vre2set.h"
 
 #define CATCHALL                                \
@@ -70,7 +71,12 @@ vre2set::compile() const
 inline bool
 vre2set::match(const char* subject) const
 {
+#ifdef HAVE_SET_MATCH_NULL_VECTOR
 	return set_->Match(subject, NULL);
+#else
+	vector<int> v;
+	return set_->Match(subject, &v);
+#endif
 }
 
 const char *