[CMake] when /usr/local doesn't exist

[Yngve Levinsen]

Dear developers,

Initial warning: This is a minor issue.

One of the computers that are submitting nightlies to my cdash server
is an iMac which does not have /usr/local created. However, it looks
like /usr/loca/lib is one of the default directories which is linked,
so his report always has a linking warning. Wouldn't it be better if
you go through the standard folders and only add them to the linker
flags if they actually exist? Of course this doesn't break anything
(at least that I'm aware of), but I sort of think it would be 'more
professional' to check this automatically... I believe he is using
cmake 2.8, unsure about which patchlevel (I can check in case it is
relevant).

Thanks for all your great work!

Cheers,
Yngve