Use -std=gnu11

Such a standard is used for linux kernel compilation. Since kbd follows
the linux kernel interfaces we can use the same standard for compiler.

Signed-off-by: Alexey Gladkov <legion@kernel.org>

configure.ac

@@ -31,6 +31,8 @@ AC_PROG_LN_S
 AC_PROG_MAKE_SET
 AC_CANONICAL_HOST
 
+AX_CHECK_COMPILE_FLAG([-std=gnu11],[CFLAGS="$CFLAGS -std=gnu11"])
+
 dnl older versions of automake didn't include this macro
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 LT_INIT

docs/process/programming-language.md

@@ -0,0 +1,34 @@
+Programming Language
+====================
+
+The kernel is written in the C programming language [c-language].
+More precisely, the kernel is typically compiled with `gcc` [gcc]
+under `-std=gnu11` [gcc-c-dialect-options]: the GNU dialect of ISO C11.
+`clang` [clang] is also supported.
+
+This dialect contains many extensions to the language [gnu-extensions],
+and many of them are used within the kbd as a matter of course.
+
+Attributes
+---------------
+
+One of the common extensions used throughout the kernel are
+attributes [gcc-attribute-syntax]. Attributes allow to introduce
+implementation-defined semantics to language entities (like variables,
+functions or types) without having to make significant syntactic changes
+to the language (e.g. adding a new keyword) [n2049].
+
+In some cases, attributes are optional (i.e. a compiler not supporting them
+should still produce proper code, even if it is slower or does not perform
+as many compile-time checks/diagnostics).
+
+Please refer to `src/libcommon/compiler_attributes.h` for more information.
+
+
+[c-language] http://www.open-std.org/jtc1/sc22/wg14/www/standards
+[gcc] https://gcc.gnu.org
+[clang] https://clang.llvm.org
+[gcc-c-dialect-options] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
+[gnu-extensions] https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
+[gcc-attribute-syntax] https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html
+[n2049] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2049.pdf