Modernize OPENSSL_COMPILE_ASSERT.

MSVC 2015 supports the static_assert keyword in C mode (not quite what C11
specifies: _Static_assert is the keyword and static_assert is a macro in
assert.h, but close enough). GCC and Clang both support _Static_assert at all C
versions. GCC has supported it in GCC 4.6.

glibc supports the assert.h macro since glibc 2.16, but does condition it on
the version, so we likely can't rely on that yet. Still, this means we should
be able to rely on proper static assertions at this point. In particular, this
means we'd no longer worry about emitting multiple typedefs of the same name.

Though at some point, it'd be nice to rely on being built in C11 mode. Then we
can just pull in assert.h and use bare static_assert, and the atomics business
needn't be a build flag.

Update-Note: If static asserts break the build, it's this CL's fault.
Change-Id: I1b09043aae41242f6d40386c063e381d00b028d8
Reviewed-on: https://boringssl-review.googlesource.com/c/32604
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/type_check.h

@@ -64,11 +64,19 @@ extern "C" {
 #endif
 
 
-#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
-#define OPENSSL_COMPILE_ASSERT(cond, msg) _Static_assert(cond, #msg)
+// TODO(davidben): |OPENSSL_COMPILE_ASSERT| used to be implemented with a
+// typedef, so the |msg| parameter is a token. It now requires C11 or C++11
+// static asserts. If this change survives to 2018-11-05, switch the parameter
+// to a string. (Maybe rename to |OPENSSL_STATIC_ASSERT| while we're at it.)
+#if defined(__cplusplus) || (defined(_MSC_VER) && !defined(__clang__))
+// In C++ and non-clang MSVC, |static_assert| is a keyword.
+#define OPENSSL_COMPILE_ASSERT(cond, msg) static_assert(cond, #msg)
 #else
-#define OPENSSL_COMPILE_ASSERT(cond, msg) \
-  typedef char OPENSSL_COMPILE_ASSERT_##msg[((cond) ? 1 : -1)] OPENSSL_UNUSED
+// C11 defines the |_Static_assert| keyword and the |static_assert| macro in
+// assert.h. While the former is available at all versions in Clang and GCC, the
+// later depends on libc and, in glibc, depends on being built in C11 mode. We
+// do not require this, for now, so use |_Static_assert| directly.
+#define OPENSSL_COMPILE_ASSERT(cond, msg) _Static_assert(cond, #msg)
 #endif