Use system-defined bswap macros where available

JSLinux headers have them, and ours caused a name conflict.

c/flisp.c

@@ -956,8 +956,11 @@ no_kw:
     return nargs;
 }
 
+#ifndef bswap_16
 #define bswap_16(x) (((x)&0x00ff) << 8 | ((x)&0xff00) >> 8)
+#endif
 
+#ifndef bswap_32
 #ifdef __INTEL_COMPILER
 #define bswap_32(x) _bswap(x)
 #else
@@ -965,6 +968,7 @@ no_kw:
     ((((x)&0xff000000) >> 24) | (((x)&0x00ff0000) >> 8) | \
      (((x)&0x0000ff00) << 8) | (((x)&0x000000ff) << 24))
 #endif
+#endif
 
 #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 #define GET_INT32(a)                                             \