Simplify STATIC_ASSERT macro.

For C++, use always __static_assert and for C, use the negative array
index.

gcc/ChangeLog:

	* basic-block.h (STATIC_ASSERT): Use normal STATIC_ASSERT.
	* system.h (STATIC_ASSERT): Define as static_assert for C++
	and fallback to array index in C.

gcc/basic-block.h

@@ -158,10 +158,7 @@ struct GTY((chain_next ("%h.next_bb"), chain_prev ("%h.prev_bb"))) basic_block_d
 /* This ensures that struct gimple_bb_info is smaller than
    struct rtl_bb_info, so that inlining the former into basic_block_def
    is the better choice.  */
-typedef int __assert_gimple_bb_smaller_rtl_bb
-              [(int) sizeof (struct rtl_bb_info)
-               - (int) sizeof (struct gimple_bb_info)];
-
+STATIC_ASSERT (sizeof (rtl_bb_info) >= sizeof (gimple_bb_info));
 
 #define BB_FREQ_MAX 10000
 

gcc/system.h

@@ -835,8 +835,7 @@ extern void fancy_abort (const char *, int, const char *)
 #define STATIC_CONSTANT_P(X) (false && (X))
 #endif
 
-/* static_assert (COND, MESSAGE) is available in C++11 onwards.  */
-#if __cplusplus >= 201103L
+#ifdef __cplusplus
 #define STATIC_ASSERT(X) \
   static_assert ((X), #X)
 #else