system/FileDescriptor: check `__linux__` instead of `__linux`

`__linux` is the deprecated non-standard macros which appears to be
not present at all on PowerPC.

Closes #398

NEWS

@@ -1,6 +1,7 @@
 ver 0.21.1 (not yet released)
 * decoder
   - ffmpeg: fix build failure with non-standard FFmpeg installation path
+* fix build failure on Linux-PowerPC
 
 ver 0.21 (2018/10/31)
 * configuration

src/system/FileDescriptor.cxx

@@ -76,7 +76,7 @@ FileDescriptor::IsSocket() const noexcept
 
 #endif
 
-#ifdef __linux
+#ifdef __linux__
 
 bool
 FileDescriptor::Open(FileDescriptor dir, const char *pathname,

src/system/FileDescriptor.hxx

@@ -116,7 +116,7 @@ public:
 		return FileDescriptor(-1);
 	}
 
-#ifdef __linux
+#ifdef __linux__
 	bool Open(FileDescriptor dir, const char *pathname,
 		  int flags, mode_t mode=0666) noexcept;
 #endif