Makefile: adopt --std=gnu11 for HOSTCFLAGS on Linux

Following the conversion of the SPDX license tags, a number of files
compiled with -pedantic now generate warnings similar to the following
for using C99-style '//' comments in ISO C90 code:

  tools/gen_eth_addr.c:1:1: warning: C++ style comments are not allowed in ISO C90
   // SPDX-License-Identifier: GPL-2.0+
   ^

The SPDX comment-style change means that these files have adopted C99,
so need to change the language-standard to --std=gnu99 or --std=gnu11
to let the compiler know this.

As we now require GCC 6 or newer for the cross-compiler, the project has
implicitly moved the project to GNU11: let older GCC versions on various
Linux distros know to treat our host tools as GNU11 as well.

References: commit 83d290c56fab ("SPDX: Convert all of our single license tags to Linux Kernel style")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

Makefile

@@ -258,6 +258,15 @@ HOSTCFLAGS   = -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer \
 		$(if $(CONFIG_TOOLS_DEBUG),-g)
 HOSTCXXFLAGS = -O2
 
+# With the move to GCC 6, we have implicitly upgraded our language
+# standard to GNU11 (see https://gcc.gnu.org/gcc-5/porting_to.html).
+# Some Linux distributions (including RHEL7, SLES13, Debian 8) still
+# have older compilers as their default, so we make it explicit for
+# these that our host tools are GNU11 (i.e. C11 w/ GNU extensions).
+ifeq ($(HOSTOS),linux)
+HOSTCFLAGS += --std=gnu11
+endif
+
 ifeq ($(HOSTOS),cygwin)
 HOSTCFLAGS	+= -ansi
 endif