[pushed] Only ignore -Wenum-compare-switch if it exists

[Simon Marchi <simon dot marchi at ericsson dot com>]

My patch

  dwarf2read: Silence -Wenum-compare-switch warning
  132448f8359a268f34f074b0908b5255b568da06

made some parts of dwarf2read.c ignore warnings about switch using enums
of different kinds.  What I did not realize was that older Clang
versions (prior to 6) did not have that warning, and therefore give this
error:

/home/emaisin/src/binutils-gdb/gdb/dwarf2read.c:24187:7: error: unknown warning group '-Wenum-compare-switch', ignored [-Werror,-Wunknown-pragmas]
      DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES
      ^
/home/emaisin/src/binutils-gdb/gdb/common/diagnostics.h:42:3: note: expanded from macro 'DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES'
  DIAGNOSTIC_IGNORE ("-Wenum-compare-switch")
  ^
/home/emaisin/src/binutils-gdb/gdb/common/diagnostics.h:27:3: note: expanded from macro 'DIAGNOSTIC_IGNORE'
  _Pragma (STRINGIFY (GCC diagnostic ignored option))
  ^
<scratch space>:10:25: note: expanded from here
 GCC diagnostic ignored "-Wenum-compare-switch"
                        ^

Clang has a way to test if it knows about a particular warning.  This
patch uses that feature to only define
DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES to something if the
warning is recognized by the Clang version being used.  I tested
building dwarf2read.c with clang 4, 5, 6, as well as gcc.

gdb/ChangeLog:

	* common/diagnostics.h
	(DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES): Only
	define if the compiler knows about -Wenum-compare-switch.
---
 gdb/ChangeLog            | 6 ++++++
 gdb/common/diagnostics.h | 9 ++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 7689e0e..edb3cd4 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,11 @@
 2017-12-30  Simon Marchi  <simon.marchi@ericsson.com>
 
+	* common/diagnostics.h
+	(DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES): Only
+	define if the compiler knows about -Wenum-compare-switch.
+
+2017-12-30  Simon Marchi  <simon.marchi@ericsson.com>
+
 	* dwarf2read.c (dwarf_decode_macro_bytes): Ignore
 	-Wenum-compare-switch warning.
 	(dwarf_decode_macros): Likewise.
diff --git a/gdb/common/diagnostics.h b/gdb/common/diagnostics.h
index 30c0fd6..c9ef6b6 100644
--- a/gdb/common/diagnostics.h
+++ b/gdb/common/diagnostics.h
@@ -38,9 +38,12 @@
   DIAGNOSTIC_IGNORE ("-Wdeprecated-register")
 # define DIAGNOSTIC_IGNORE_UNUSED_FUNCTION \
   DIAGNOSTIC_IGNORE ("-Wunused-function")
-# define DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES \
-  DIAGNOSTIC_IGNORE ("-Wenum-compare-switch")
-
+# if __has_warning ("-Wenum-compare-switch")
+#  define DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES \
+   DIAGNOSTIC_IGNORE ("-Wenum-compare-switch")
+# else
+#  define DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES
+# endif
 #elif defined (__GNUC__) /* GCC */
 
 # define DIAGNOSTIC_IGNORE_SELF_MOVE
-- 
2.7.4