Only use intrinsics when the compiler supports them.
https://github.com/cisco/ChezScheme/issues/845
https://github.com/cisco/ChezScheme/commit/f1ad314a3809074c6f2b986a5db6a84ad88700c1
--- c/atomic.h.orig	2024-02-05 16:52:07.000000000 -0600
+++ c/atomic.h	2024-06-27 00:56:20.000000000 -0500
@@ -114,7 +114,7 @@
 }
 # define CAS_LOAD_ACQUIRE(a, old, new) S_cas_any_fence(1, a, old, new)
 # define CAS_STORE_RELEASE(a, old, new) S_cas_any_fence(0, a, old, new)
-#elif (__GNUC__ >= 5) || defined(__clang__)
+#elif (__GNUC__ >= 5) || C_COMPILER_HAS_BUILTIN(__sync_bool_compare_and_swap)
 # define CAS_ANY_FENCE(a, old, new) __sync_bool_compare_and_swap((ptr *)(a), TO_PTR(old), TO_PTR(new))
 #elif defined(__i386__) || defined(__x86_64__)
 # if ptr_bits == 64
--- c/pb.h.orig
+++ c/pb.h
@@ -71,12 +71,25 @@ enum {
 
 #define SIGN_FLIP(r, a, b) ((~((a ^ b) | (r ^ ~b))) >> (ptr_bits-1))
 
-#if (__GNUC__ >= 5) || defined(__clang__)
+#if C_COMPILER_HAS_BUILTIN(__builtin_add_overflow) \
+  && C_COMPILER_HAS_BUILTIN(__builtin_sub_overflow) \
+  && C_COMPILER_HAS_BUILTIN(__builtin_mul_overflow)
+# define USE_OVERFLOW_INTRINSICS 1
+#elif (__GNUC__ >= 5)
 # define USE_OVERFLOW_INTRINSICS 1
 #else
 # define USE_OVERFLOW_INTRINSICS 0
 #endif
 
+#if C_COMPILER_HAS_BUILTIN(__builtin_bswap16) \
+  && C_COMPILER_HAS_BUILTIN(__builtin_bswap32)
+# define USE_BSWAP_INTRINSICS 1
+#elif (__GNUC__ >= 5)
+# define USE_BSWAP_INTRINSICS 1
+#else
+# define USE_BSWAP_INTRINSICS 0
+#endif
+
 /* Use `machine_state * RESTRICT_PTR`, because machine registers won't
    be modified in any way other than through the machine-state pointer */
 
@@ -714,7 +727,7 @@ enum {
 #if ptr_bits == 64
 #define doi_pb_rev_op_pb_int16_pb_register(instr) \
   do_pb_rev_op_pb_int16_pb_register(INSTR_dr_dest(instr), INSTR_dr_reg(instr))
-# if USE_OVERFLOW_INTRINSICS
+# if USE_BSWAP_INTRINSICS
 /* See note below on unsigned swap. */
 #  define do_pb_rev_op_pb_int16_pb_register(dest, reg) \
   regs[dest] = ((uptr)(((iptr)((uptr)__builtin_bswap16(regs[reg]) << 48)) >> 48))
@@ -740,7 +753,7 @@ enum {
 #if ptr_bits == 64
 # define doi_pb_rev_op_pb_int32_pb_register(instr) \
    do_pb_rev_op_pb_int32_pb_register(INSTR_dr_dest(instr), INSTR_dr_reg(instr))
-# if USE_OVERFLOW_INTRINSICS
+# if USE_BSWAP_INTRINSICS
 /* x86_64 GCC before 12.2 incorrectly compiles the code below to an unsigned swap.
    Defeat that by using the unsigned-swap intrinsic (which is good, anyway), then
    shift up and back. */
--- c/version.h.orig
+++ c/version.h
@@ -33,6 +33,14 @@
 #define FORCEINLINE static inline
 #endif
 
+/* GCC 10 and later and all versions of Clang provide `__has_builtin` for
+   checking for builtins. */
+#ifdef __has_builtin
+# define C_COMPILER_HAS_BUILTIN(x) __has_builtin(x)
+#else
+# define C_COMPILER_HAS_BUILTIN(x) 0
+#endif
+
 /*****************************************/
 /* Architectures                         */