updated readme
reviewer changes
Update top-level README.txt and installation guide
to describe the new directory structure.
fallback to system pkg-config if necessary
Trac #16273: update README

URL: http://trac.sagemath.org/16273
Reported by: vbraun
Ticket author(s): Volker Braun, John Palmieri
Reviewer(s): William Stein, Volker Braun
fix permisions, reviewer patches
export variables
fix search path, add tests
avoid potential race when creating pkg-config
fix indentation
skip checks if we do not use the system pkg-config
fix grep
Trac #16274: include system pkgconfig path

Here we actually use the system-wide `pkg-config` if present to query
both Sage's as well as the system's package-config (`*.pc`) files.

If only Sage's `pkgconf` is available, `.pc` files (and hence the
packages they belong to) installed outside of Sage still won't be seen
by Sage's `pkg-config` wrapper script.

It is therefore still recommended to have a system-wide `pkg-config`,
despite Sage now having its own, e.g. to let R use graphics packages
Sage doesn't ship.

URL: http://trac.sagemath.org/16274
Reported by: vbraun
Ticket author(s): Volker Braun
Reviewer(s): Leif Leonhardy