Use __linux__, not only __linux, to detect Linux

According to https://sourceforge.net/p/predef/wiki/OperatingSystems/,
this is the macro specified by POSIX.

This change actually fixes failure to compile on the ppc64le version of
Red Hat Enterprise Linux 8. The fact that this is architecture-dependent
is probably a bug in that distribution.

src/c4/platform.hpp

@@ -23,7 +23,7 @@
 #   else
 #       error "Unknown Apple platform"
 #   endif
-#elif defined(__linux)
+#elif defined(__linux__) || defined(__linux)
 #   define C4_UNIX
 #   define C4_LINUX
 #elif defined(__unix)
@@ -36,7 +36,7 @@
 #   error "unknown platform"
 #endif
 
-#if defined(__posix) || defined(__unix__) || defined(__linux)
+#if defined(__posix) || defined(__unix__) || defined(C4_LINUX)
 #   define C4_POSIX
 #endif