Implement su_errno and su_seterrno as functions

This avoid warnings about expressions with unused values.  These
warnings break the build with clang.

An optimizing compiler should still inline these calls.  If that turns
out not to happen on some platform, we could rename the functions used
internally and declare them static inline.

libs/sofia-sip/libsofia-sip-ua/su/sofia-sip/su_errno.h

@@ -53,11 +53,6 @@ SOFIAPUBFUN int su_errno(void);
 /** Set the su error. */
 SOFIAPUBFUN int su_seterrno(int);
 
-#if !SU_HAVE_WINSOCK
-#define su_errno() (errno)
-#define su_seterrno(n) ((errno = (n)), -1)
-#endif
-
 #if defined(__APPLE_CC__) || defined(__OpenBSD__) || defined(__NetBSD__) || defined(__FreeBSD__)
 #ifndef EBADMSG
 #define EBADMSG EFAULT

libs/sofia-sip/libsofia-sip-ua/su/su_errno.c

@@ -157,4 +157,7 @@ const char *su_strerror(int errcode)
   return strerror(errcode);
 }
 
+int su_errno(void) { return errno; }
+int su_seterrno(int n) { errno = n; return -1; }
+
 #endif /* SU_HAVE_WINSOCK */