Add more variables to be used during compilation

For compilers:
* %{compiler}% : the name of the compiler (not the switch)

For packages:
* %{compiler}% : the name of the compiler (not the switch)
* %{package}% : the name of the package being compiled
* %{version}% : the version of the package being compiled
* %{hash}% : if a source archive is available, the digest of that archive
* %{depends}% : the comma-separated list of this package dependencies

Also add a '.' in front of temp files (stdout, stderr, etc.) to make them invisible to build tools.