Commit 8efede20ef ("Use _Static_assert") introduced the usage of
_Static_assert(). However, _Static_assert() is a fairly new thing,
since it was introduced only in gcc 4.6. In order to support older
compilers, this patch adds a configure.in test that checks whether
_Static_assert() is usable or not, and adjust the behavior of the
assert_cc() macro accordingly.

Signed-off-by: Thomas Petazzoni <redacted>
---
Changes since v1:
 * When _Static_assert() is not available, rely on the previously used
   sizeof() trick, as suggested by Lucas de Marchi.
---
 configure.ac    | 6 ++++++
 libkmod/macro.h | 5 +++++
 2 files changed, 11 insertions(+)

diff --git a/configure.ac b/configure.ac
index 40e54cf..cbe12f3 100644
--- a/configure.ac
+++ b/configure.ac

@@ -52,6 +52,12 @@ AC_CHECK_MEMBERS([struct stat.st_mtim], [], [], [#include <sys/stat.h>])
 # Check kernel headers
 AC_CHECK_HEADERS_ONCE([linux/module.h])

+AC_MSG_CHECKING([whether _Static_assert() is supported])
+AC_COMPILE_IFELSE(
+       [AC_LANG_SOURCE([[_Static_assert(1, "Test");]])],
+        [AC_DEFINE([HAVE_STATIC_ASSERT], [1], [Define is _Static_assert() is available])