build: fix pkg-config output parsing in configure

Fix parsing of `pkg-config --cflags-only-I`.  The configure_library()
step sometimes appended a list in a list instead of list of strings to
include_dirs.

This commit removes the default handling for includes and libpath
options.  They don't have defaults at the moment and I don't see that
changing anytime soon.  Fixing the code is more work and because it's
dead code anyway, I opted to remove it instead.

Fixes: https://github.com/nodejs/io.js/issues/1985
PR-URL: https://github.com/nodejs/io.js/pull/1986


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>