system/FileDescriptor: make pipe2() mandatory on Linux

configure.ac

@@ -241,7 +241,7 @@ AC_SEARCH_LIBS([socket], [network socket])
 AC_SEARCH_LIBS([gethostbyname], [nsl])
 
 if test x$host_is_linux = xyes; then
-	AC_CHECK_FUNCS(pipe2 accept4 linkat)
+	AC_CHECK_FUNCS(accept4 linkat)
 fi
 
 AC_CHECK_FUNCS(getpwnam_r getpwuid_r)

src/system/FileDescriptor.cxx

@@ -120,7 +120,7 @@ FileDescriptor::CreatePipe(FileDescriptor &r, FileDescriptor &w) noexcept
 {
 	int fds[2];
 
-#ifdef HAVE_PIPE2
+#ifdef __linux__
 	const int flags = O_CLOEXEC;
 	const int result = pipe2(fds, flags);
 #elif defined(_WIN32)
@@ -145,7 +145,7 @@ FileDescriptor::CreatePipeNonBlock(FileDescriptor &r,
 {
 	int fds[2];
 
-#ifdef HAVE_PIPE2
+#ifdef __linux__
 	const int flags = O_CLOEXEC|O_NONBLOCK;
 	const int result = pipe2(fds, flags);
 #else
@@ -158,7 +158,7 @@ FileDescriptor::CreatePipeNonBlock(FileDescriptor &r,
 	r = FileDescriptor(fds[0]);
 	w = FileDescriptor(fds[1]);
 
-#ifndef HAVE_PIPE2
+#ifndef __linux__
 	r.SetNonBlocking();
 	w.SetNonBlocking();
 #endif