build: Better handle zlib-specific C flags

There are two warnings that we enforce for our own code that zlib does
not. There's also the visibility attribute that we check at configure
time. And regarding the visibility attribute, zlib no longer relies on
a NO_VIZ macro and aligned with the autoconf naming convention and wants
HAVE_HIDDEN instead.

Conflicts:
	configure.ac
	lib/libvgz/Makefile.am

configure.ac

@@ -244,29 +244,25 @@ AC_CHECK_FUNCS([pthread_setname_np])
 AC_CHECK_FUNCS([pthread_mutex_isowned_np])
 LIBS="${save_LIBS}"
 
-# Support for visibility attribute
-save_CFLAGS="${CFLAGS}"
-CFLAGS="${CFLAGS} -Werror"
+# zlib-specific flags
+libvgz_extra_cflags="-Wno-error=strict-prototypes"
 
+dnl https://github.com/madler/zlib/issues/633
+libvgz_extra_cflags="$libvgz_extra_cflags -Wno-error=deprecated-non-prototype"
+
+AC_SUBST(libvgz_extra_cflags)
+
+# Support for visibility attribute (zlib)
 AC_CACHE_CHECK([whether we have support for visibility attributes],
 	[ac_cv_have_viz],
-	[AC_RUN_IFELSE(
+	[AC_COMPILE_IFELSE(
 		[AC_LANG_PROGRAM([[
-			#if ((__GNUC__-0) * 10 + __GNUC_MINOR__-0 >= 33)
-			#  define ZLIB_INTERNAL __attribute__((visibility ("hidden")))
-			#else
-			#  define ZLIB_INTERNAL
-			#endif
-			int ZLIB_INTERNAL foo;
+			int __attribute__((visibility ("hidden"))) foo;
 		]],[])],
-	[ac_cv_have_viz=yes],
-	[ac_cv_have_viz=no])
+	[AC_DEFINE([HAVE_HIDDEN], [1],
+		[Define to 1 if visibility attribute hidden is available.])
+	])
 ])
-if test "$ac_cv_have_viz" = no; then
-	libvgz_extra_cflags="-DNO_VIZ"
-	AC_SUBST(libvgz_extra_cflags)
-fi
-CFLAGS="${save_CFLAGS}"
 
 if test "x$GCC" = "xyes"; then
 	libvgz_extra_cflags="${libvgz_extra_cflags} -Wno-unknown-warning-option -Wno-implicit-fallthrough"

lib/libvgz/Makefile.am

 #
 
-AM_LDFLAGS  = $(AM_LT_LDFLAGS)
+AM_CFLAGS = $(AM_LT_CFLAGS) \
+	-D_LARGEFILE64_SOURCE=1 \
+	-DZLIB_CONST \
+	$(libvgz_extra_cflags) \
+	@SAN_CFLAGS@
 
-noinst_LIBRARIES = libvgz.a
+AM_LDFLAGS = $(AM_LT_LDFLAGS)
 
-libvgz_a_CFLAGS = -D_LARGEFILE64_SOURCE=1 -DZLIB_CONST \
-	$(libvgz_extra_cflags) @SAN_CFLAGS@
+noinst_LIBRARIES = libvgz.a
 
 libvgz_a_SOURCES = \
 	adler32.c \