Make NEST installation independant from source & build directory

[tammoippen]

In NEST config.h we set the source and build directory and use it within NEST (#define NEST_SOURCEDIR "@PKGSRCDIR@" and #define NEST_BUILDDIR "@PKGBUILDDIR@"). When we want to create pre-build binaries for different platforms, e.g. for OS X or for Debian, we cannot rely on the presence of our sources or build directory. The aim of this issue is to gather all dependancies of the installed NEST on the source / build directory and have them removed. As a start one could remove these two defines, work through all the compile errors and see, if make installcheck is still good with NEST.

Errors, I already observe with installcheck: with an installed NEST, this should work as expected, but at least two tests will fail, when the sources are not present:

• regressiontests/ticket-659-copyright.py as its want to check all sources for correct copyright notice
• unittests/test_round_validate.sli: the sli function validate goes through all the help files and looks for examples, which it executes and compares to the expected result in the example. But, before it regenerates the help from the Source: file stated in the help file, which will fail when no sources are available.

[heplesser]

This was addressed by #318.