Add support for ccache and improve pg_configure for non-copy mode

When pg_compile is used in non-copy mode, the root tree of the git folder
where call is being done from is used for the configure phase. This allows
calls of pg_compile to be done inside a postgres git tree.

Also, support for ccache is added with this commit with the addition of
a new option -e that needs to specify a sub ccache name to designate a
folder in HOME_CCACHE_DIR. This option is not mandatory and let at the
convenience of the user.
Ignore .ccache entries
Add git alias for exec

This allows to launch commands from the root tree of a git folder like:
git exec make
This is pretty useful to compile a code tree without moving back to the
root of build.