intltool: be more clear on what is the default podir
intltool: make podir default value consistent

Is more predictable if intltool_po and intltool_in have the same default
podir, that is, the current src directory.
intltool: fix INTLCACHE directory
intltool: don't install merged files by default in intltool_in

It doesn't make sense to install merged files in LOCALEDIR in most
cases.
intltool: move some code outside a loop in intltool_in

It's fine to execute this code just one time since the environment
variables won't change throughout the different source files.