Merge pull request #5 from chouqin/ctags

add ctags to gitignore
cache projects/LLVMBuild.txt
Merge pull request #6 from chouqin/build

cache projects/LLVMBuild.txt
set llc executable's name to llc-3.4 to distinguish it from the origin
source
Merge pull request #7 from chouqin/build

set llc executable's name to llc-3.4 to distinguish it from the origin