Squashed commit of the following:

commit b0aa2076563116983866345114cd65ab828bb19a
Author: Attila Oláh <attilaolah@gmail.com>
Date:   Fri Jul 25 09:14:40 2014 +0200

    use GO_GIT_DESCRIBE_SYMBOL

commit dc5e568fcb669a5dd2cd692c4899cb2dbede1403
Merge: cd3ebcd 43f7b5f
Author: Attila Oláh <attilaolah@gmail.com>
Date:   Wed Jul 23 12:50:20 2014 +0200

    Merge branch 'master' of git://github.com/kr/heroku-buildpack-go

    Conflicts:
    	bin/compile

commit cd3ebcd125a1ab19ea3076e259883218af8514d7
Author: Attila Oláh <attilaolah@gmail.com>
Date:   Mon Jul 21 10:44:38 2014 +0200

    export GIT_DIR as HEROKU_GIT_DIR so the makefile can use it

commit c770426d3e9913fbefeed6a56c15e9c71e691bd8
Author: Attila Oláh <attilaolah@gmail.com>
Date:   Fri Jul 18 21:07:38 2014 +0200

    but only if there is a heroku target

commit 948938dfcee14b0f0c57b6c17281ad3058c07513
Author: Attila Oláh <attilaolah@gmail.com>
Date:   Fri Jul 18 20:49:11 2014 +0200

    allow custom build process if there is a makefile