CMake: Explicitly enable POSIX extensions

This is needed when compiling on non-Linux platforms, such as Amiga,
Switch, etc.

CMakeLists.txt

@@ -326,6 +326,11 @@ if(WIN32)
   endif()
 endif()
 
+if(NOT WIN32 AND NOT APPLE)
+  # Enable POSIX extensions such as `readlink` and `ftruncate`.
+  add_definitions(-D_POSIX_C_SOURCE=200809L)
+endif()
+
 if(HAIKU)
   target_link_libraries(devilutionx PRIVATE network)
 endif()