Move LYX_(BEGIN|END)_MUTE_GCC_WARNING macros to their own header.

It was not nice to other build systems to define that in configure.ac.

configure.ac

@@ -313,32 +313,6 @@ 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

@@ -15,6 +15,7 @@
 #include <map>
 #include <string>
 
+#include "support/mute_warning.h"
 
 namespace lyx {
 

src/WordList.h

@@ -13,6 +13,7 @@
 #define WORDLIST_H
 
 #include "support/docstring.h"
+#include "support/mute_warning.h"
 
 #include <memory>
 

src/frontends/FontMetrics.h

@@ -14,6 +14,8 @@
 #ifndef FONT_METRICS_H
 #define FONT_METRICS_H
 
+
+#include "support/mute_warning.h"
 #include "support/strfwd.h"
 
 #include <vector>

src/frontends/qt/GuiApplication.cpp

@@ -71,6 +71,7 @@
 #include "support/lassert.h"
 #include "support/lstrings.h"
 #include "support/lyxalgo.h" // sorted
+#include "support/mute_warning.h"
 #include "support/textutils.h"
 #include "support/Messages.h"
 #include "support/os.h"

src/support/Makefile.am

@@ -85,6 +85,7 @@ liblyxsupport_a_SOURCES = \
 	lyxmagic.h \
 	lyxtime.cpp \
 	lyxtime.h \
+	mute_warning.h \
 	mutex.h \
 	mutex.cpp \
 	Messages.cpp \