Avoid dangling-reference warning in several places

This new warning in gcc 13 is annoying because it happens in certain
parts of our code where it is harmless to pass a temporary variable to
a function that returns a reference.

This patch introduces a new pair of macros,
LYX_BEGIN_MUTE_GCC_WARNING(warn) and LYX_END_MUTE_GCC_WARNING, which
can be used to define a block of code where a given GCC warning is disabled.

The macros are no-ops with compilers other than gcc, although some
compilers that pretend to be GCC make be mis-detected. The worse that
can happen AFAIU is a bunch of warnings.

The macro relies on an intimidating set of for nested macros. The goal
of these macros is to build a nested string bit by bit. Here is how it
works:

PRAGMA_IGNORE(dangling-reference)
  => PRAGMA_IGNORE_1(-Wdangling-reference)
  => PRAGMA_IGNORE_2("-Wdangling-reference")
  => PRAGMA_IGNORE_3(GCC diagnostic ignored "-Wdangling-reference")
  => _Pragma("GCC diagnostic ignored \""-Wdangling-reference\"")

The next question is: what is _Pragma() good for? Well, it is a
version of #pragma that can be used in a macro.

And finally, what are those pragmas good for? The 'push' and 'pop'
ones make changes to warnings local. The 'ignored' ones allow
to disable some warnings. And disabling -Wpragmas ensures that we do
not have a warning if we try to disable a warning that is not
supported by the compiler.

configure.ac

@@ -313,6 +313,33 @@ char * strerror(int n);
 #  define USE_WCHAR_T
 #endif
 
+#if defined(__GNUC__) && !defined(__clang__)
+/* This macro can be used to stipulate that a given GCC warning is not
+ * relevant in a given block.
+ *
+ * The -Wpragmas bit takes care of the case where -W<warn> is not implemented
+ *
+ * The idea for PRAGMA_IGNORE has been stolen from
+ * https://stackoverflow.com/questions/45762357/how-to-concatenate-strings-in-the-arguments-of-pragma#comment124444258_45783809
+ * The difficulty is to put the <warn> value inside nested quotes; it is done
+ * using nested macros.
+ */
+#  define PRAGMA_IGNORE(x) PRAGMA_IGNORE_1(-W##x)
+#  define PRAGMA_IGNORE_1(x) PRAGMA_IGNORE_2(#x)
+#  define PRAGMA_IGNORE_2(x) PRAGMA_IGNORE_3(GCC diagnostic ignored x)
+#  define PRAGMA_IGNORE_3(x) _Pragma(#x)
+#  define LYX_BEGIN_MUTE_GCC_WARNING(warn)		\
+  _Pragma("GCC diagnostic push") \
+  _Pragma("GCC diagnostic ignored \"-Wpragmas\"") \
+  PRAGMA_IGNORE(warn)
+#  define LYX_END_MUTE_GCC_WARNING \
+  _Pragma("GCC diagnostic pop")
+#else
+#  define LYX_BEGIN_MUTE_GCC_WARNING(warn)
+#  define LYX_END_MUTE_GCC_WARNING
+#endif
+
+
 #endif
 ])
 

src/Mover.h

@@ -181,8 +181,10 @@ private:
 
 
 extern Movers & theMovers();
+LYX_BEGIN_MUTE_GCC_WARNING(dangling-reference)
 /// @c returns the Mover registered for format @c fmt.
 extern Mover const & getMover(std::string  const & fmt);
+LYX_END_MUTE_GCC_WARNING
 /** Register a specialised @c command to be used to copy a file
  *  of format @c fmt.
  */

src/WordList.h

@@ -36,7 +36,9 @@ private:
 	std::unique_ptr<Impl> d;
 };
 
+LYX_BEGIN_MUTE_GCC_WARNING(dangling-reference)
 WordList & theWordList(std::string const & lang);
+LYX_END_MUTE_GCC_WARNING
 
 } // namespace lyx
 

src/frontends/FontMetrics.h

@@ -177,8 +177,12 @@ class Font;
 class FontInfo;
 
 /// Implementation is in Application.cpp
+
+LYX_BEGIN_MUTE_GCC_WARNING(dangling-reference)
 frontend::FontMetrics const & theFontMetrics(Font const & f);
 frontend::FontMetrics const & theFontMetrics(FontInfo const & fi);
+LYX_END_MUTE_GCC_WARNING
+
 
 } // namespace lyx