Debian/kFreeBSD should use the FreeBSD-specific codepaths, not generic Unix.
--- a/src/physfs_platforms.h Wed Jul 12 20:39:03 2017 -0400
+++ b/src/physfs_platforms.h Wed Jul 12 20:49:53 2017 -0400
@@ -50,7 +50,7 @@
# define PHYSFS_PLATFORM_SOLARIS 1
# define PHYSFS_PLATFORM_UNIX 1
# define PHYSFS_PLATFORM_POSIX 1
-#elif defined(__FreeBSD__) || defined(__DragonFly__)
+#elif defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || defined(__DragonFly__)
# define PHYSFS_PLATFORM_FREEBSD 1
# define PHYSFS_PLATFORM_BSD 1
# define PHYSFS_PLATFORM_UNIX 1