--copy-dt-needed-entries breaks with gold

[sithhell]

The "--copy-dt-needed-entries" linker option breaks when using the gold linker instead of the older ld linker on linux. This is very problematic as newer linux distributions (debian and ubuntu as of my knowledge now) ship with gold as the default linker. That means /usr/bin/ld is gold.

I suggest to remove this flag entirely. My proposed solution is to expose a HPX_LIBRARIES cmake variable in FindHPX.cmake which should be used to add to target_link_libraries in external builds. This is standard procedure for any other library out there.

Comments? Suggestions?

[sithhell]

Fixed with commits:
7d869d2
fc2bc41
4f0ad70