Portable use of bswap_32

configure.ac

@@ -72,6 +72,7 @@ AM_CONDITIONAL(OS_QNX, test "$os_qnx" = "true")
 LT_INIT([disable-static win32-dll pic-only])
 AC_CHECK_HEADERS([ \
     arpa/inet.h \
+    byteswap.h \
     errno.h \
     fcntl.h \
     limits.h \

src/modbus-data.c

@@ -24,10 +24,35 @@
 #endif
 #include <string.h>
 #include <assert.h>
-#include <byteswap.h>
 
 #include "modbus.h"
 
+#if defined(HAVE_BYTESWAP_H)
+#  include <byteswap.h>
+#endif
+
+#if defined(__GNUC__)
+#  define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__ * 10)
+#  if GCC_VERSION >= 430
+// Since GCC >= 4.30, GCC provides __builtin_bswapXX() alternatives so we switch to them
+#    undef bswap_32
+#    define bswap_32 __builtin_bswap32
+#  endif
+#endif
+
+#if !defined (bswap_16) || !defined (bswap_32)
+#   warning "Fallback on C functions for bswap_32"
+static inline uint16_t bswap_16(uint16_t x)
+{
+    return (x >> 8) | (x << 8);
+}
+
+static inline uint32_t bswap_32(uint32_t x)
+{
+    return (bswap_16(x & 0xffff) << 16) | (bswap_16(x >> 16));
+}
+#endif
+
 /* Sets many bits from a single byte value (all 8 bits of the byte value are
    set) */
 void modbus_set_bits_from_byte(uint8_t *dest, int index, const uint8_t value)