[PATCH v2] gcc-plugins: fix gcc 11 indigestion with plugins...

["Valdis Klētnieks" <valdis.kletnieks@vt.edu>]

Fedora Rawhide has started including gcc 11,and the g++ compiler
throws a wobbly when it hits scripts/gcc-plugins:

  HOSTCXX scripts/gcc-plugins/latent_entropy_plugin.so
In file included from /usr/include/c++/11/type_traits:35,
                 from /usr/lib/gcc/x86_64-redhat-linux/11/plugin/include/system.h:244,
                 from /usr/lib/gcc/x86_64-redhat-linux/11/plugin/include/gcc-plugin.h:28,
                 from scripts/gcc-plugins/gcc-common.h:7,
                 from scripts/gcc-plugins/latent_entropy_plugin.c:78:
/usr/include/c++/11/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO
 C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
   32 | #error This file requires compiler and library support \

Patch is more complicated than would otherwise be needed, because
older gcc (4.9, 5.4) have gnu++11 but throw an error due to a gcc bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69959

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
---
diff --git a/scripts/gcc-plugins/Makefile b/scripts/gcc-plugins/Makefile
index d66949bfeba4..cc779973724a 100644
--- a/scripts/gcc-plugins/Makefile
+++ b/scripts/gcc-plugins/Makefile
@@ -21,10 +21,13 @@ always-y += $(GCC_PLUGIN)
 
 GCC_PLUGINS_DIR = $(shell $(CC) -print-file-name=plugin)
 
+# need gnu++11 for gcc 11, but 4.9 and 5.4 need gnu++98
+GCC_FLAVOR = $(call cc-ifversion, -ge, 1100, 11, 98)
+
 plugin_cxxflags	= -Wp,-MMD,$(depfile) $(KBUILD_HOSTCXXFLAGS) -fPIC \
-		   -I $(GCC_PLUGINS_DIR)/include -I $(obj) -std=gnu++98 \
+		   -I $(GCC_PLUGINS_DIR)/include -I $(obj) -std=gnu++$(GCC_FLAVOR) \
 		   -fno-rtti -fno-exceptions -fasynchronous-unwind-tables \
-		   -ggdb -Wno-narrowing -Wno-unused-variable -Wno-c++11-compat \
+		   -ggdb -Wno-narrowing -Wno-unused-variable \
 		   -Wno-format-diag
 
 plugin_ldflags	= -shared