[intro.{r,d}efs] Make "C standard library" a defined term

Partially fixes ISO/CS-002 (C++23 DIS).

Author: tkoeppe

source/intro.tex

@@ -70,16 +70,6 @@
 Available from: \url{https://www.unicode.org/versions/latest/}
 \end{itemize}
 
-\pnum
-The library described in ISO/IEC 9899:2018, Clause 7,
-is hereinafter called the
-\defnx{C standard library}{C!standard library}.
-\begin{footnote}
-With the qualifications noted in \ref{\firstlibchapter}
-through \ref{\lastlibchapter} and in \ref{diff.library}, the C standard
-library is a subset of the \Cpp{} standard library.
-\end{footnote}
-
 \pnum
 The operating system interface described in ISO/IEC 9945:2009 is
 hereinafter called \defn{POSIX}.
@@ -164,6 +154,15 @@
 \defncontext{statement}
 compound statement
 
+\indexdefn{C!standard library}%
+\definition{C standard library}{defns.c.lib}
+library described in \IsoC{}, Clause 7
+\begin{defnote}
+With the qualifications noted in \ref{\firstlibchapter}
+through \ref{\lastlibchapter} and in \ref{diff.library},
+the C standard library is a subset of the \Cpp{} standard library.
+\end{defnote}
+
 \definition{character}{defns.character}
 \indexdefn{character}%
 \defncontext{library}