Add -Wextra to C and C++ compilation, and silence some warnings that

it uncovered.

src/Makefile.am

 AUTOMAKE_OPTIONS = subdir-objects
 
-AM_CPPFLAGS = @VMOD_INCLUDES@ -Wall -Werror
+AM_CPPFLAGS = @VMOD_INCLUDES@ -Wall -Werror -Wextra
 CFLAGS += -std=c99
-AM_CXXFLAGS = -Wall -Werror -std=c++11 @RE2_CFLAGS@
+AM_CXXFLAGS = -Wall -Werror -Wextra -std=c++11 @RE2_CFLAGS@
 
 vmoddir = @VMOD_DIR@
 vmod_LTLIBRARIES = libvmod_re2.la

src/vre2/vre2.cpp

@@ -63,7 +63,7 @@ vre2::match(const char *subject, const size_t len, const int ngroups,
 	return re_->Match(subject, 0, len, RE2::UNANCHORED, groups, ngroups);
 }
 
-inline const int
+inline int
 vre2::ngroups() const
 {
 	return re_->NumberOfCapturingGroups();
@@ -130,7 +130,7 @@ vre2_init(vre2 **vre2p, const char *pattern, unsigned utf8,
 	CATCHALL
 }
 
-const size_t
+size_t
 vre2_matchsz(void)
 {
 	return sizeof(StringPiece);

src/vre2/vre2.h

@@ -47,7 +47,7 @@ public:
 	virtual ~vre2();
 	bool match(const char *subject, size_t len, int ngroups,
 		   StringPiece* groups) const;
-	const int ngroups() const;
+	int ngroups() const;
 	int get_group(const char * const name) const;
 	bool replace(string *text, const char * const rewrite) const;
 	bool global_replace(string *text, const char * const rewrite) const;
@@ -76,7 +76,7 @@ extern "C" {
 			      unsigned perl_classes, unsigned word_boundary,
 			      unsigned one_line);
 	const char *vre2_fini(vre2 **vre2);
-	const size_t vre2_matchsz(void);
+	size_t vre2_matchsz(void);
 	const char *vre2_ngroups(vre2 *vre2, int * const ngroups);
 	const char *vre2_match(vre2 *vre2, const char * const subject,
 			       size_t len, int * const match, int ngroups,