Fix build for Ubuntu 18.10

[KeenS]

idris-posix fails to build on my machine. According to this QA, S_ISDIR and S_ISREG is more portable.

detailed info

Error Message

Entering directory `./src'
Type checking ./Main.idr
In file included from /tmp/idris68312-0.c:3:
/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/libs/posix/stat_wrappers.c: In function ‘idris_posix_stat_file_type’:
/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/libs/posix/stat_wrappers.c:6:27: error: ‘S_IFMT’ undeclared (first use in this function); did
you mean ‘S_IXOTH’?
   return buffer.st_mode & S_IFMT;
                           ^~~~~~
                           S_IXOTH
/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/libs/posix/stat_wrappers.c:6:27: note: each undeclared identifier is reported only once for each function it appears in
FAILURE: "cc" ["-O1","-foptimize-sibling-calls","-fwrapv","-fno-strict-overflow","-std=c99","-pipe","-fdata-sections","-ffunction-sections","-D_POSIX_C_SOURCE=200809L","-DHAS_PTHREAD","-DIDRIS_ENABLE_STATS","-I.","-Wl,-gc-sections","/tmp/idris68312-0.c","-L/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/rts","-lidris_rts","-lgmp","-lpthread","-I/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/rts","-lm","-I.","-I/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/libs/posix","-I/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/libs/prelude","-I/home/shun/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.3.0/libs/base","-I.","-I/home/kim/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.2.0/libs/prelude","-I/home/kim/compile/Idris-dev/.stack-work/install/x86_64-linux/lts-11.3/8.2.2/share/x86_64-linux-ghc-8.2.2/idris-1.2.0/libs/base","-o","/home/shun/Idris/idris-posix-sandbox/main"]
Leaving directory `./src'

Versions

$ gcc --version
gcc (Ubuntu 8.2.0-7ubuntu1) 8.2.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ idris --version
1.3.0-git:61cf812e
$ uname -a
Linux fractal 4.18.0-11-generic #12-Ubuntu SMP Tue Oct 23 19:22:37 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux