[3.14] gh-135504: Document LIBZSTD_CFLAGS and LIBZSTD_LIBS config options (GH-135505) (#135515)

Author: miss-islington

Doc/using/configure.rst

@@ -445,6 +445,14 @@ Options for third-party dependencies
    C compiler and linker flags for ``libuuid``, used by :mod:`uuid` module,
    overriding ``pkg-config``.
 
+.. option:: LIBZSTD_CFLAGS
+.. option:: LIBZSTD_LIBS
+
+   C compiler and linker flags for ``libzstd``, used by :mod:`compression.zstd` module,
+   overriding ``pkg-config``.
+
+   .. versionadded:: 3.14
+
 .. option:: PANEL_CFLAGS
 .. option:: PANEL_LIBS