OS X: Add RPATH support for Mac.

RPATH support is activated on targets that have the MACOSX_RPATH
property turned on.
For install time, it is also useful to set INSTALL_RPATH to help
find dependent libraries with an @rpath in their install name.

Also adding detection of rpath conflicts when using frameworks.
Xcode: Add rpath support in Xcode generator.
OS X: Add support for @rpath in export files.

Also expand the IMPORTED_SONAME property for targets
to match the install_name.
OS X: Add test for rpaths on Mac.

This also tests rpaths through export/import.
Merge topic 'rpath-on-mac' into next

f22eccc OS X: Add test for rpaths on Mac.
d6cebae OS X: Add support for @rpath in export files.
526641c Xcode: Add rpath support in Xcode generator.
f53971d OS X: Add RPATH support for Mac.