curl-gnumake.sh: re-sync builds with cmake/autotools [ci skip]

- enable symbol hiding manually to match cmake/autotools.

- enable variadic macros. These macros became required in curl 8.3.0
  with curl/curl#11421.
  These macros will no longer be needed with curl 8.5.0
  after curl/curl#12167.

- set `HAVE_SNPRINTF`. curl's `config-win32.h` missed to set it,
  causing a minor difference in the generated binaries compared to
  cmake/autotools builds.
  Ref: curl/curl#12325

With these changes GNU Make builds are in sync again with
cmake/autotools.

curl-gnumake.sh

@@ -40,6 +40,13 @@ _VER="$1"
   export LDFLAGS="${_LDFLAGS_GLOBAL}"
   export LIBS=''
 
+  CFLAGS+=' -fvisibility=hidden'
+
+  if [ "${CURL_VER_}" = '8.4.0' ]; then
+    CPPFLAGS+=' -DHAVE_VARIADIC_MACROS_C99=1 -DHAVE_VARIADIC_MACROS_GCC=1'
+    CPPFLAGS+=' -DHAVE_SNPRINTF=1'
+  fi
+
   # Picky compiler warnings as seen in curl CMake/autotools.
   # builds with llvm/clang 15 and gcc 12.2:
   #   https://clang.llvm.org/docs/DiagnosticsReference.html