build: Revisit zlib C flags one more time

Trying to fix the build for clang 15 actually broke the build for GCC.

The -Werror that was initially set after saving CFLAGS was meant to be
part of NO_VIZ test. We turn warnings into errors later in the configure
script so at this point we shouldn't care about it. If we really do, we
can move this check below the line where -Werror is set.

GCC chokes on -Wno-error=deprecated-non-prototype so instead we add it
conditionally.

To match the naming convention everywhere else, libvgz_extra_cflags was
renamed to VGZ_CFLAGS.

Refs 118fd10c

configure.ac

@@ -249,19 +249,14 @@ LIBS="${save_LIBS}"
 
 AC_CHECK_DECL([__SUNPRO_C], [SUNCC="yes"], [SUNCC="no"])
 
-if test "$SUNCC" = "yes" ; then
-	CFLAGS="${CFLAGS} -errwarn=%all,no%E_EMPTY_TRANSLATION_UNIT"
-else
-	CFLAGS="${CFLAGS} -Werror"
-fi
-
 # zlib-specific flags
-libvgz_extra_cflags="-Wno-error=strict-prototypes -Wno-unknown-warning-option"
+VGZ_CFLAGS="-Wno-error=strict-prototypes -Wno-unknown-warning-option"
 
 dnl https://github.com/madler/zlib/issues/633
-libvgz_extra_cflags="$libvgz_extra_cflags -Wno-error=deprecated-non-prototype"
+AX_CHECK_COMPILE_FLAG([-Wno-error=deprecated-non-prototype],
+	[VGZ_CFLAGS="$VGZ_CFLAGS -Wno-error=deprecated-non-prototype"])
 
-AC_SUBST(libvgz_extra_cflags)
+AC_SUBST(VGZ_CFLAGS)
 
 # Support for visibility attribute (zlib)
 AC_CACHE_CHECK([whether we have support for visibility attributes],

lib/libvgz/Makefile.am

@@ -3,7 +3,7 @@
 AM_CFLAGS = $(AM_LT_CFLAGS) \
 	-D_LARGEFILE64_SOURCE=1 \
 	-DZLIB_CONST \
-	$(libvgz_extra_cflags)
+	$(VGZ_CFLAGS)
 
 AM_LDFLAGS = $(AM_LT_LDFLAGS)