add https://github.com/gitignore/Mercury.gitignore to src/
src/Mercury.options: Add MAIN_TARGET = libmercury_misc
Merge pull request #1 from sebgod/with-main-target

src/Mercury.options: Add MAIN_TARGET = libmercury_misc
Merge pull request #2 from sebgod/with-gitignore

add github/gitignore Mercury.gitignore to src/