include/qemu/bswap.h: Handle being included outside extern "C" block

Make bswap.h handle being included outside an 'extern "C"' block:
all system headers are included first, then all declarations are
put inside an 'extern "C"' block.

This requires a little rearrangement as currently we have an ifdef
ladder that has some system includes and some local declarations
or definitions, and we need to separate those out.

We want to do this because dis-asm.h includes bswap.h, dis-asm.h
may need to be included from C++ files, and system headers should
not be included within 'extern "C"' blocks.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

include/qemu/bswap.h

@@ -1,8 +1,6 @@
 #ifndef BSWAP_H
 #define BSWAP_H
 
-#include "fpu/softfloat-types.h"
-
 #ifdef CONFIG_MACHINE_BSWAP_H
 # include <sys/endian.h>
 # include <machine/bswap.h>
@@ -12,7 +10,18 @@
 # include <endian.h>
 #elif defined(CONFIG_BYTESWAP_H)
 # include <byteswap.h>
+#define BSWAP_FROM_BYTESWAP
+# else
+#define BSWAP_FROM_FALLBACKS
+#endif /* ! CONFIG_MACHINE_BSWAP_H */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "fpu/softfloat-types.h"
+
+#ifdef BSWAP_FROM_BYTESWAP
 static inline uint16_t bswap16(uint16_t x)
 {
     return bswap_16(x);
@@ -27,7 +36,9 @@ static inline uint64_t bswap64(uint64_t x)
 {
     return bswap_64(x);
 }
-# else
+#endif
+
+#ifdef BSWAP_FROM_FALLBACKS
 static inline uint16_t bswap16(uint16_t x)
 {
     return (((x & 0x00ff) << 8) |
@@ -53,7 +64,10 @@ static inline uint64_t bswap64(uint64_t x)
             ((x & 0x00ff000000000000ULL) >> 40) |
             ((x & 0xff00000000000000ULL) >> 56));
 }
-#endif /* ! CONFIG_MACHINE_BSWAP_H */
+#endif
+
+#undef BSWAP_FROM_BYTESWAP
+#undef BSWAP_FROM_FALLBACKS
 
 static inline void bswap16s(uint16_t *s)
 {
@@ -494,4 +508,8 @@ DO_STN_LDN_P(be)
 #undef le_bswaps
 #undef be_bswaps
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* BSWAP_H */