ext/gsl/extconf.rb: Remove narray.so from the list of files to link. (a8ce8f0, bb15af3)

Reverts a8ce8f0 and bb15af3 to accommodate for temporary build directories such as on Heroku.

Closes #5. Reopens #4.
ext/gsl/extconf.rb: Refactored and extracted helper method gsl_gem_config.
ext/gsl/extconf.rb (gsl_gem_config): Add <target>.so to the list of files to link. (e4ce8b4)

Instead of basing the decision on the platform as in a8ce8f0, look for relevant flags.

Closes #4.
ChangeLog: Cleanup.