fallback: remove sysconf fallback

sysconf was introduced in IEEE Std 1003.1-1988 (POSIX.1) and exists
on every system I can find.

Work on #2999.

configure.ac

@@ -313,7 +313,7 @@ AC_STRUCT_DIRENT_D_TYPE
 AC_CHECK_FUNCS( wcsdup wcsndup wcslen wcscasecmp wcsncasecmp fwprintf )
 AC_CHECK_FUNCS( futimes wcwidth wcswidth wcstok fputwc fgetwc )
 AC_CHECK_FUNCS( wcstol wcslcat wcslcpy lrand48_r killpg )
-AC_CHECK_FUNCS( backtrace backtrace_symbols_fd sysconf getifaddrs )
+AC_CHECK_FUNCS( backtrace backtrace_symbols_fd getifaddrs )
 AC_CHECK_FUNCS( futimens clock_gettime )
 
 AC_CHECK_DECL( [mkostemp], [ AC_CHECK_FUNCS([mkostemp]) ] )

src/fallback.cpp

@@ -955,19 +955,6 @@ int backtrace(void **buffer, int size) { return 0; }
 char **backtrace_symbols_fd(void *const *buffer, int size, int fd) { return 0; }
 #endif
 
-#ifndef HAVE_SYSCONF
-
-long sysconf(int name) {
-    if (name == _SC_ARG_MAX) {
-#ifdef ARG_MAX
-        return ARG_MAX;
-#endif
-    }
-
-    return -1;
-}
-#endif
-
 #ifndef HAVE_NAN
 double nan(char *tagp) { return 0.0 / 0.0; }
 #endif

src/fallback.h

@@ -253,11 +253,6 @@ extern int _nl_msg_cat_cntr;
 int killpg(int pgr, int sig);
 #endif
 
-#ifndef HAVE_SYSCONF
-#define _SC_ARG_MAX 1
-long sysconf(int name);
-#endif
-
 #ifndef HAVE_NAN
 double nan(char *tagp);
 #endif