build: use "-std=gnu++11" as compiler flags

So far, we didn't specify "-std" option, and it was ill defined which C
behavior we get.

Note that we already use __attribute__((__cleanup__(fcn))), so
effectively we require either gcc or clang. A standard C compiler won't
do, and that is for good reasons, because those gcc-isms are too good to
avoid.

Also, it's 2024. It's basically impossilbe that you use gcc/clang with a
version that doesn't support (gcc's) C11 variant. So standardize on
that. I don't think we currently require newer C features, so this is
sufficient for us and for now.

Makefile.am

@@ -32,6 +32,10 @@ pkgconfig_DATA = \
 	libnl-xfrm-3.0.pc \
 	$(NULL)
 
+extra_cppflags = \
+	-std=gnu++11 \
+	$(NULL)
+
 warn_cppflags = \
 	-Wall \
 	-Wextra \
@@ -394,6 +398,7 @@ default_includes = \
 	$(NULL)
 
 default_cppflags = \
+	$(extra_cppflags) \
 	$(warn_cppflags) \
 	$(defines_cppflags) \
 	$(default_includes) \