configure.ac: add automake conditional "LINUX"

src/system/EpollFD.cxx

@@ -28,7 +28,6 @@
  */
 
 #include "config.h"
-#ifdef USE_EPOLL
 #include "EpollFD.hxx"
 #include "Error.hxx"
 
@@ -38,5 +37,3 @@ EpollFD::EpollFD()
 	if (!fd.IsDefined())
 		throw MakeErrno("epoll_create1() failed");
 }
-
-#endif /* USE_EPOLL */

src/system/EventFD.cxx

@@ -28,7 +28,6 @@
  */
 
 #include "config.h"
-#ifdef __linux__
 #include "EventFD.hxx"
 #include "system/Error.hxx"
 #include "util/Compiler.h"
@@ -60,5 +59,3 @@ EventFD::Write() noexcept
 	gcc_unused ssize_t nbytes =
 		fd.Write(&value, sizeof(value));
 }
-
-#endif /* USE_EVENTFD */

src/system/SignalFD.cxx

@@ -28,7 +28,6 @@
  */
 
 #include "config.h"
-#ifdef __linux__
 #include "SignalFD.hxx"
 #include "Error.hxx"
 
@@ -52,5 +51,3 @@ SignalFD::Read() noexcept
 		? info.ssi_signo
 		: -1;
 }
-
-#endif /* __linux__ */