Added gcc dependency for Compiling and installing grit section

[magic890]

Added explicit dependency on README for gcc.
It is required for running cabal install gitit as stated here: #500

[jgm]

Well, this is debatable. As I understand it, ghc comes with a version of gcc.
So I doubt that a separate gcc install should be required in normal circumstances. The setup in #500 was problematic, but until knowing more about why I don't want to make such changes.

[magic890]

Ok, now that we have clarified the setup related to #500, and it is working, we can investigate on this behaviour.
As reminder, I'm running it on CentOS 6, with GHC 7.6.3 and cabal-1.16.0.2.

If you run ghc-pkg list it will prompt all the packages installed with ghc.
Mine output:

ghc-pkg list
/usr/hs/ghc/7.6.3/lib/ghc-7.6.3/package.conf.d
   Cabal-1.16.0
   array-0.4.0.1
   base-4.6.0.1
   bin-package-db-0.0.0.0
   binary-0.5.1.1
   bytestring-0.10.0.2
   containers-0.5.0.0
   deepseq-1.3.0.1
   directory-1.2.0.1
   filepath-1.3.0.1
   ghc-7.6.3
   ghc-prim-0.3.0.0
   haskell2010-1.1.1.0
   haskell98-2.0.0.2
   hoopl-3.9.0.0
   hpc-0.6.0.0
   integer-gmp-0.5.0.0
   old-locale-1.0.0.5
   old-time-1.1.0.1
   pretty-1.1.1.0
   process-1.1.0.2
   rts-1.0
   template-haskell-2.8.0.0
   time-1.4.0.1
   unix-2.6.0.1
/root/.ghc/x86_64-linux-7.6.3/package.conf.d
   ConfigFile-1.1.4
   Diff-0.3.2
   HStringTemplate-0.7.3
   HTTP-4000.2.20
   HUnit-1.2.5.2
   JuicyPixels-3.2.5.2
   MissingH-1.3.0.1
   SHA-1.6.4.1
   aeson-0.7.0.6
   asn1-encoding-0.9.0
   asn1-parse-0.9.1
   asn1-types-0.3.0
   async-2.0.2
   attoparsec-0.13.0.0
   base64-bytestring-1.0.0.1
   blaze-builder-0.4.0.1
   blaze-html-0.8.0.2
   blaze-markup-0.7.0.2
   byteable-0.1.1
   bytestring-builder-0.10.6.0.0
   case-insensitive-1.2.0.4
   cereal-0.4.1.1
   cmark-0.3.4
   conduit-1.2.4.2
   connection-0.2.5
   cookie-0.4.1.6
   cryptohash-0.11.6
   cryptonite-0.5
   css-text-0.1.2.1
   data-default-0.5.3
   data-default-class-0.0.1
   data-default-instances-base-0.0.1
   data-default-instances-containers-0.0.1
   data-default-instances-dlist-0.0.1
   data-default-instances-old-locale-0.0.1
   deepseq-generics-0.1.1.2
   digest-0.0.1.2
   dlist-0.7.1.1
   enclosed-exceptions-1.0.1.1
   exceptions-0.8.0.2
   extensible-exceptions-0.1.1.4
   feed-0.3.9.7
   filemanip-0.3.6.3
   filestore-0.6.0.6
   ghc-paths-0.1.0.9
   gitit-0.10.7
   haddock-library-1.2.0
   happstack-server-7.4.4
   hashable-1.2.3.2
   highlighting-kate-0.6
   hoauth2-0.4.8
   hourglass-0.2.9
   hslogger-1.2.9
   hslua-0.3.13
   html-1.0.1.2
   http-client-0.4.12
   http-client-tls-0.2.2
   http-conduit-2.1.5
   http-types-0.8.6
   json-0.9.1
   lifted-base-0.2.3.6
   memory-0.7
   mime-types-0.1.0.6
   mmorph-1.0.4
   monad-control-1.0.0.4
   mtl-2.2.1
   nats-1
   network-2.6.2.0
   network-info-0.2.0.6
   network-uri-2.6.0.3
   pandoc-1.14.0.4
   pandoc-types-1.12.4.4
   parsec-3.1.9
   pem-0.2.2
   primitive-0.6
   random-1.1
   recaptcha-0.1.0.3
   regex-base-0.93.2
   regex-compat-0.95.1
   regex-pcre-builtin-0.94.4.8.8.35
   regex-posix-0.95.2
   resourcet-1.1.5
   safe-0.3.9
   scientific-0.3.3.8
   semigroups-0.16.2.2
   sendfile-0.7.9
   socks-0.5.4
   split-0.2.2
   stm-2.4.4
   streaming-commons-0.1.12.1
   syb-0.5.1
   system-filepath-0.4.13.4
   tagsoup-0.13.3
   temporary-1.2.0.3
   texmath-0.8.2
   text-1.2.1.1
   threads-0.5.1.3
   time-compat-0.1.0.3
   time-locale-compat-0.1.1.0
   tls-1.3.1
   transformers-0.4.3.0
   transformers-base-0.4.4
   transformers-compat-0.4.0.4
   unix-compat-0.4.1.4
   unordered-containers-0.2.5.1
   uri-0.1.6.3
   url-2.1.3
   utf8-string-0.3.8
   uuid-1.3.10
   uuid-types-1.0.1
   vector-0.10.12.3
   void-0.7
   x509-1.6.0
   x509-store-1.6.0
   x509-system-1.6.0
   x509-validation-1.6.0
   xhtml-3000.2.1
   xml-1.3.14
   xss-sanitize-0.3.5.5
   yaml-0.8.11
   zip-archive-0.2.3.7
   zlib-0.6.1.1

As you can see I don't have gcc bundled with ghc.

I have installed it manually via yum install gcc.x86_64.

[jgm]

+++ Matteo Guarnerio [Jun 29 15 13:10 ]:

As you can see I don't have gcc bundled with ghc.

It wouldn't show up in this list. This is a list of
libraries, not tools.